https://mail.gnome.org/archives/networkmanager-list/2018-May/msg00000.html

https://bugzilla.gnome.org/show_bug.cgi?id=794658

On D-Bus level, string (s) or object paths (o) cannot be NULL.
Thus, whenver server exposes such an object, it gets automatically
coerced to "" or "/", respectively.

On client side, libnm should coerce certain properties back, for which
"" is just not a sensible value.

For example, an empty NM_DEVICE_ETHERNET_HW_ADDRESS should be instead
exposed as NULL.

Technically, this is an API change. However, all users were well advised
to expect both NULL and "" as possible return values and handle them
accordingly.

- All internal source files (except "examples", which are not internal)
  should include "config.h" first. As also all internal source
  files should include "nm-default.h", let "config.h" be included
  by "nm-default.h" and include "nm-default.h" as first in every
  source file.
  We already wanted to include "nm-default.h" before other headers
  because it might contains some fixes (like "nm-glib.h" compatibility)
  that is required first.

- After including "nm-default.h", we optinally allow for including the
  corresponding header file for the source file at hand. The idea
  is to ensure that each header file is self contained.

- Don't include "config.h" or "nm-default.h" in any header file
  (except "nm-sd-adapt.h"). Public headers anyway must not include
  these headers, and internal headers are never included after
  "nm-default.h", as of the first previous point.

- Include all internal headers with quotes instead of angle brackets.
  In practice it doesn't matter, because in our public headers we must
  include other headers with angle brackets. As we use our public
  headers also to compile our interal source files, effectively the
  result must be the same. Still do it for consistency.

- Except for <config.h> itself. Include it with angle brackets as suggested by
  https://www.gnu.org/software/autoconf/manual/autoconf.html#Configuration-Headers

Document that the GPtrArray returned by

  - nm_device_get_lldp_neighbors()
  - nm_ip_config_get_addresses()
  - nm_ip_config_get_routes()

is immutable and can be used by callers without the need of a copy.

Cleans up all of the warnings that aren't overly annoying to clean up.

config.h should be included from every .c file, and it should be
included before any other include. Fix that.

(As a side effect of how I did this, this also changes us to
consistently use "config.h" rather than <config.h>. To the extent that
it matters [which is not much], quotes are more correct anyway, since
we're talking about a file in our own build tree, not a system
include.)

Create NMIPConfig as the parent of NMIP4Config and NMIP6Config, and
remove the two subclasses from the public API; while it's convenient
to still have both internally, they are now identical to the outside
world.