We have "src/libNetworkManager.la" which is an intermediate static
library containing everything that ends up in "src/NetworkManager".

Likewise, add "libnm/liblibnm.la" to be the static library that contains
everything from "libnm/libnm.la".

The point of these libraries is to tie everything together that is used
by "src/NetworkManager" and "libnm/libnm.la" so that it also can be used
by unit-tests. Thereby, the unit tests will link statically against the
code of libnm. The problem is that the unit tests also want to access
internal functionality of libnm that is not accessible when dynamically
linking.

In part, this new library replaces "libnm/libnm-utils.la". The previous
name was confusing, because to us everything is an "utils", and it's
unclear what the purpose of that library was. Now the purpose should be
a bit clearer: liblibnm.la is a step before libnm.la, similar to what
libNetworkManager.la is to NetworkManager.