Fix runtime on FreeBSD >=10.0

Gajim relies on nslookup when looking up for an SRV record of jabber server.
Since, nslookup was removed from FreeBSD 10, gajim is now fails to connect
unless jabber-server hostname and port were defined by user explicitly. This
patch make it use host(1) for this task.

Submitted by: walker_643 <walker_643@yahoo.com> (by email)
MFH: 2014Q1