platform: link management

platform: link management

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.