And move some code from NMLinuxPlatform to NMPlatform, where it belongs.

The advantage is that we reuse (and test!) the NMPCache implementation for
tracking addresses.

Also, we now always expose proper NMPObjects from both linux and fake
platform.

For example,

  obj = NMP_OBJECT_UP_CAST (nm_platform_ip4_address_get (...));

will work as expected. Also, the caller is now by NMPlatform API
allowed to take and keep a reference to the returned objects.