Remove unreachable trust-checking code and associated machinery

There used to be a time when cf-serverd checked IP reported by the
client and did some trust calculations based on that (including
denying connections from NATed hosts, because NATed hosts didn't
know their IP address as it is seen from cf-serverd side).

In order to overcome NAT issue "skipverify" option has been introduced, to
exclude some IPs/networks from IP/DNS lookup checks.

In CFEngine 3.0 this code became unreachable and stayed like this until today,
and nobody noticed. Hence, it's safe to remove it and deprecate "skipverify"
option.

Note that github interface is not very good for reviewing diffs, so I suggest you to get this branch and do a
git diff --patience.