openssh: GSSAPI key exchange patch

This brings [Simon Wilkinson's GSSAPI key exchange patch][1] to 6.2p2, which is also included in Apple's OpenSSH along with Kerberos.  It adds GSSAPIKeyExchange, GSSAPIClientIdentity, GSSAPIServerIdentity, GSSAPITrustDns, GSSAPIRenewalForcesRekey, GSSAPITrustDns keywords to ssh config.

[1]: http://www.sxw.org.uk/computing/patches/openssh.html

Complements #182.