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

Closes: #225.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>
openssh: add option to enable GSSAPI key exchange patch.

* Other stylenit.