[DNS] zone import should strip trailing .@ from relative record names

The following is a legitimate DNS record from a BIND zone file:

_sip._tls.@	3600 	IN	SRV	100	1	443	sipdir.online.lync.com

Note the '.@' in the record name, which currently causes import to fail (Bad Request).

Where a record name ends in .@, we should strip it off (and then apply the current value of $ORIGIN).

