MEDIUM: resolvers/dns: split dns.c into dns.c and resolvers.c

MEDIUM: resolvers/dns: split dns.c into dns.c and resolvers.c

This patch splits current dns.c into two files:

The first dns.c contains code related to DNS message exchange over UDP
and in future other TCP. We try to remove depencies to resolving
to make it usable by other stuff as DNS load balancing.

The new resolvers.c inherit of the code specific to the actual
resolvers.

Note:
It was really difficult to obtain a clean diff dur to the amount
of moved code.

Note2:
Counters and stuff related to stats is not cleany separated because
currently counters for both layers are merged and hard to separate
for now.