New functions to compare two instances of NMPlatformIP4Address, NMPlatformIP6Address,
NMPlatformIP4Route, NMPlatformIP6Route, respectively.

These functions return -1, 0 or 1 as result of the comparison. This is similar to
strcmp with the additional restriction, that only one of these 3 values will be
returned.

Signed-off-by: Thomas Haller <thaller@redhat.com>