For addresses (NMPlatformIPAddress) the @addr_source field is ignored
on a platform level. That is, all addresses inside the platform cache
have this value set to NM_IP_CONFIG_SOURCE_KERNEL. Maybe, for that reason,
the source should not be a part of the NMPlatformIPAddress structure, but
it is convenient for users to piggy back the source inside the platform
address structure.

For routes, the source is stored in NMPlatformIPRoute's @rt_source
field. When adding a route to kernel, we set the @rtm_protocol of the
route depending on the source. However, we want to map different source
values to the same protocol value.

On the other hand, when kernel sends us a route that gets put inside
the cache, we must preserve the protocol value and must not map
different protocol values to the same source.
The reason is, that a user can add two routes that only differ by
@rtm_protocol. In that sense, the @rtm_protocol fields is part of the
unique ID of a kernel route, and thus different values must map to
different sources.

Fix this, by extending the range of NMIPConfigSource to contain
a range of protocol fields.