MEDIUM: resolvers: add support of tcp address on nameserver line.

MEDIUM: resolvers: add support of tcp address on nameserver line.

This patch re-works configuration parsing, it removes the "server"
lines from "resolvers" sections introduced in commit 56fc5d9eb:
MEDIUM: resolvers: add supports of TCP nameservers in resolvers.

It also extends the nameserver lines to support stream server
addresses such as:

resolvers
  nameserver localhost tcp@127.0.0.1:53

Doing so, a part of nameserver's init code was factorized in
function 'parse_resolvers' and removed from 'post_parse_resolvers'.