TRQ-2594. Port using the munge API.

Lukasz Flis contributed making munge use the API to the 2.5 series of
code, and I am getting this into our newer codebases now.
Fix a bug in the code ported for libmunge
Make address caching more robust.

If looking up an address in the cache fails, make the function attempt
to resolve the bad ip address and return it if it can resolve it, only
failing if it can't truly be looked up.

Conflicts:

	src/lib/Libnet/net_cache.c
Fix a mistake in the previous check-in.