Add support for DNS SRV records

	adds preliminary support for DNS SRV records in a new
	function connect_srv(domain=None). If domain is None
	a lookup on the domain of the host is attempted. Otherwise
	domain is a DNS domain name to query for the SRV records.
	change service to IANA-assigned

Signed-off-by: Jan-Piet Mens <jpmens@gmail.com>