Tag addresses and routes with their source.  We'll use this later to do
(or not do) operations based on where the item came from.

One thing to note is that when synchronizing items with the kernel, all
items are read as source=KERNEL even when they originally came from
NetworkManager, since the kernel has no way of providing this source
information.  This requires the source 'priority', which
nm_ip*_config_add_address() and nm_ip*_config_add_route() must respect
to ensure that NM-owned routes don't have their source overwritten
when merging various IP configs in ip*_config_merge_and_apply().

Also of note is that memcmp() can no longer be used to compare
addresses/routes in nm-platform.c, but this had problems before
anyway with ifindex, so that workaround from nm_platform_ip4_route_sync()
can be removed.

https://bugzilla.gnome.org/show_bug.cgi?id=722843
https://bugzilla.redhat.com/show_bug.cgi?id=1005416