Replace the usage of g_str_hash() with our own nm_str_hash().

GLib's g_str_hash() uses djb2 hashing function, just like we
do at the moment. The only difference is, that we use a diffrent
seed value.

Note, that we initialize the hash seed with random data (by calling
getrandom() or reading /dev/urandom). That is a change compared to
before.

This change of the hashing function and accessing the random pool
might be undesired for libnm/libnm-core. Hence, the change is not
done there as it possibly changes behavior for public API. Maybe
we should do that later though.

At this point, there isn't much of a change. This patch becomes
interesting, if we decide to use a different hashing algorithm.