There is no general purpose file for platform utilities.
We only have nm-platform.h, which contains (mostly) functions
that operate on a NMPlatform instance (and that can be mocked
using NMFakePlatform).

Add a new file for independent utility functions. nm-platform-utils.c
should not call into functions having a NMPlatform instance, to
have them independent from platform caching and the platform
singleton.

Extend nm-platform to support IPv4 and IPv6 address management.

Address features:

* Retrieve the list of IPv4 and IPv6 addresses
* Add/delete/lookup IPv4 and IPv6 addresses
* Flush all non-linklocal addresses

Create the new nm-platform framework and implement link (or interface)
management. The nm-platform serves as the point of contact between
the rest of NetworkManager and the operating system.

There are two backends for nm-platform:

* NMFakePlatform: Fake kernel backend for testing purposes
* NMLinuxPlatform: Linux kernel backend for actual use

A comprehensive testsuite is included and will be extended with new
feature additions. To enable the Linux part of the testsuite, use
--enable-tests=root configure options and run 'make check' as root.
Use --enable-code-coverage for code coverage support.

  ./autogen.sh --enable-tests=root --enable-code-coverage
  make
  make -C src/platform check-code-coverage

Link features:

* Retrieve the list of links
* Translate between indexes and names
* Discover device type
* Add/remove dummy interfaces (for testing)

Thanks to Thomas Graf for helping with libnl3 synchronization issues.