Rework platform object cache to use NMDedupMultiIndex.

Already previously, NMPCache used NMMultiIndex and had thus
O(1) for most operations. What is new is:

- Contrary to NMMultiIndex, NMDedupMultiIndex preserves the order of
  the cached items. That is crucial to handle routes properly as kernel
  will replace the first matching route based on network/plen/metric
  properties. See related bug rh#1337855.
  Without tracking the order of routes as they are exposed
  by kernel, we cannot properly maintain the route cache.

- All NMPObject instances are now treated immutable, refcounted
  and get de-duplicated via NMDedupMultiIndex. This allows
  to have a global NMDedupMultiIndex that can be shared with
  NMIP4Config and NMRouteManager. It also allows to share the
  objects themselves.
  Immutable objects are so much nicer. We can get rid of the
  update pre-hook callback, which was required previously because
  we would mutate the object inplace. Now, we can just update
  the cache, and compare obj_old and obj_new after the fact.

- NMMultiIndex was treated as an internal of NMPCache. On the other
  hand, NMDedupMultiIndex exposes NMDedupMultiHeadEntry, which is
  basically an object that allows to iterate over all related
  objects. That means, we can now lookup objects in the cache
  and give the NMDedupMultiHeadEntry instance to the caller,
  which then can iterate the list on it's own -- without need
  for copying anything.
  Currently, at various places we still create copies of lookup
  results. That can be improved later.

The ability to share NMPObject instances should enable us to
significantly improve performance and scale with large number
of routes.

Of course there is a memory overhead of having an index for each list
entry. Each NMPObject may also require an NMDedupMultiEntry,
NMDedupMultiHeadEntry, and NMDedupMultiBox item, which are tracked
in a GHashTable. Optimally, one NMDedupMultiHeadEntry is the head
for multiple objects, and NMDedupMultiBox is able to deduplicate several
NMPObjects, so that there is a net saving.
Also, each object type has several indexes of type NMPCacheIdType.
So, worst case an NMPlatformIP4Route in the platform cache is tracked
by 8 NMPCacheIdType indexes, for each we require a NMDedupMultiEntry,
plus the shared NMDedupMultiHeadEntry. The NMDedupMultiBox instance
is shared between the 8 indexes (and possibly other).

NMPlatform, NMRouteManager and NMDefaultRouteManager are singletons
instances. Users of those are for example NMDevice, which registers
to GObject signals of both NMPlatform and NMRouteManager.

Hence, as NMDevice:dispose() disconnects the signal handlers, it must
ensure that those singleton instances live longer then the NMDevice
instance. That is usually accomplished by having users of singleton
instances own a reference to those instances.
For NMDevice that effectively means that it shall own a reference to
several singletons.

NMPlatform, NMRouteManager, and NMDefaultRouteManager are all
per-namespace. In general it doesn't make sense to have more then
one instances of these per name space. Nnote that currently we don't
support multiple namespaces yet. If we will ever support multiple
namespaces, then a NMDevice would have a reference to all of these
manager instances. Hence, introduce a new class NMNetns which bundles
them together.

(cherry picked from commit 0af2f5c28b7646c687b5180a45a9124d62c0dfac)

- use _NM_GET_PRIVATE() and _NM_GET_PRIVATE_PTR() everywhere.

- reorder statements, to have GObject related functions (init, dispose,
  constructed) at the bottom of each file and in a consistent order w.r.t.
  each other.

- unify whitespaces in signal and properties declarations.

- use NM_GOBJECT_PROPERTIES_DEFINE() and _notify()

- drop unused signal slots in class structures

- drop unused header files for device factories

sed 's#^/\*\{5\}\*\+/$#/*****************************************************************************/#' $(git grep -l '\*\{5\}' | grep '\.[hc]$') -i

Give a direct access to the cache to lookup values there. This is only
useful for testing.

"nm-logging.h" is a basic core module that should have few other
dependencies. Instead of letting nm-logging.c directly call a function
from "nm-linux-platform.c", let platform register a handler as needed.

This way, you can build a core library containing nm-logging but no
nm-platform.

When debug-logging for platform is enabled, every access to sysctl
is cached (to log the last values).

This cache can grow quite large if the system has a large number of
interfaces (e.g. docker creating veth pairs for each container).

We already used to clear the cache, when we were about to access
sysctl *and* logging was disabled in the meantime.

Now, when logging setup changes, immediately clear the cache.
Having "nm-logging.c" call into platform code is a bit of a hack
and a better design would be to have logging code emit a signal to
which platform would subscribe. But that seems to involve much
more code (especially, as no other users care about such a signal
and because nm-logging is not a GObject).

Also, log a warning when the cache grows large to inform the user
about the cache and what he can do to clear it. The extra effort to
clear the cache when changing logging setup is done so that we do
what we tell the user: changing the logging level, will clear the
cache -- right away, not some time later when the next message is
logged.

Previously, src/nm-ip4-config.h, libnm/nm-ip4-config.h, and
libnm-glib/nm-ip4-config.h all used "NM_IP4_CONFIG_H" as an include
guard, which meant that nm-test-utils.h could not tell which of them
was being included (and so, eg, if you tried to include
nm-ip4-config.h in a libnm test, it would fail to compile because
nm-test-utils.h was referring to symbols in src/nm-ip4-config.h).

Fix this by changing the include guards in the non-API-stable parts of
the tree:

  - libnm-glib/nm-ip4-config.h remains   NM_IP4_CONFIG_H
  - libnm/nm-ip4-config.h now uses     __NM_IP4_CONFIG_H__
  - src/nm-ip4-config.h now uses       __NETWORKMANAGER_IP4_CONFIG_H__

And likewise for all other headers.

The two non-"nm"-prefixed headers, libnm/NetworkManager.h and
src/NetworkManagerUtils.h are now __NETWORKMANAGER_H__ and
__NETWORKMANAGER_UTILS_H__ respectively, which, while not entirely
consistent with the general scheme, do still mostly make sense in
isolation.