In practice, this should only matter when there are multiple
header files with the same name. That is something we try
to avoid already, by giving headers a distinct name.

When building NetworkManager itself, we clearly want to use
double-quotes for including our own headers.
But we also want to do that in our public headers. For example:

  ./a.c
    #include <stdio.h>
    #include <nm-1.h>
    void main() {
        printf ("INCLUDED %s/nm-2.h\n", SYMB);
    }

  ./1/nm-1.h
    #include <nm-2.h>

  ./1/nm-2.h
    #define SYMB "1"

  ./2/nm-2.h
    #define SYMB "2"

$ cc -I./2 -I./1 ./a.c
$ ./a.out
INCLUDED 2/nm-2.h

Exceptions to this are
  - headers in "shared/nm-utils" that include <NetworkManager.h>. These
    headers are copied into projects and hence used like headers owned by
    those projects.
  - examples/C

This is required to add objects in the "Types and Values" section and
in the API index. Later, we may want to add useful content in those
empty comments.

As with the settings, each device type was defining its own error
type, containing either redundant or non-useful error codes. Drop all
of the subtype-specific errors, and reduce things to just
NM_DEVICE_ERROR_FAILED, NM_DEVICE_ERROR_INCOMPATIBLE_CONNECTION, and
NM_DEVICE_ERROR_INVALID_CONNECTION.

The device-type-specific errors were only returned from their
nm_device_connection_compatible() implementations, so this is also a
good opportunity to simplify those, by moving duplicated functionality
into the base NMDevice implementation, and then allowing the
subclasses to assume that the connection has already been validated in
their own code. Most of the implementations now just check that the
connection has the correct type for the device (which can't be done at
the NMDevice level since some device types (eg, Ethernet) support
multiple connection types.)

Also, make sure that all of the error messages are localized.

Add nm-types.h defining all the type structs, to avoid future include
loops. Clean up the includes in all of the installed headers.

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.

Add NetworkManager.h, which includes all of the other NM header, and
require all external users of libnm to use that rather than the
individual headers.

(An exception is made for nm-dbus-interface.h,
nm-vpn-dbus-interface.h, and nm-version.h, which can be included
separately.)

Add reserved slots to those classes that were missing them (or had run
out), and sync up the number of slots across classes:

  - 8 slots for "important" classes, abstract base classes, and
    classes we expect we might need to add new virtual methods or
    signals to later.

  - 4 for everything else

Also, rearrange the class elements in a few places into standard order
(signals first, then methods).

This commit begins creating the new "libnm", which will replace
libnm-util and libnm-glib.

The main reason for the libnm-util/libnm-glib split is that the daemon
needs to link to libnm-util (to get NMSettings, NMConnection, etc),
but can't link to libnm-glib (because it uses many of the same type
names as the NetworkManager daemon. eg, NMDevice). So the daemon links
to only libnm-util, but basically all clients link to both.

With libnm, there will be only a single client-visible library, and
NetworkManager will internally link against a private "libnm-core"
containing the parts that used to be in libnm-util.

(The "libnm-core" parts still need to be in their own directory so
that the daemon can see those header files without also seeing the
ones in libnm/ that conflict with its own headers.)

[This commit just copies the source code from libnm-util/ to
libnm-core/, and libnm-glib/ to libnm/:

  mkdir -p libnm-core/tests/
  mkdir -p libnm/tests/
  cp libnm-util/*.[ch] libnm-util/nm-version.h.in libnm-core/
  rm -f libnm-core/nm-version.h libnm-core/nm-setting-template.[ch] libnm-core/nm-utils-enum-types.[ch]
  cp libnm-util/tests/*.[ch] libnm-core/tests/
  cp libnm-glib/*.[ch] libnm/
  rm -f libnm/libnm_glib.[ch] libnm/libnm-glib-test.c libnm/nm-glib-enum-types.[ch]
  cp libnm-glib/tests/*.[ch] libnm/tests/

]