Add support for SRV records.

Add support for SRV records.

There is a patch for djbdns that adds support for SRV records to both
tinydns-data and axfr-get:

  From: Michael Handler <handler@sub-rosa.com>
  To: dns@list.cr.yp.to
  Subject: tinydns-data SRV & axfr-get SRV/PTR patches
  Date: Thu, 14 Sep 2000 20:37:50 -040

Many distributions carry the patch, but valtz rejects the SRV records
because it doesn't recognize the "S" indicator or know how to validate
the port, weight, or priority fields.

This commit adds support for the new record type, and adds validation
routines for the three new fields. All of them are the same: ports,
weights, and priorities are all integers between 0 and 65536.