sshfp (ssh+dns) code updated

[Jakob Schlyter]

hi,

I recently committed an update of the code that handles lookup of SSHFP
resource records in DNS. this code is now included by default, the old DNS
and DNSSEC defines has been removed.

for more information, read about VerifyHostKeyDNS in ssh_config(5) and
check out README.dns.


feedback would be appreciated,

	jakob