openssh: Add gssapi patch used by other major distros

This patch is borrowed verbatim from Debian, where it is actively
maintained for each openssh update.  It's also included in Fedora's
openssh package, in Arch linux as openssh-gssapi in the AUR, in MacOS
X, and presumably various other platforms and linux distros.

The main relevant parts of this patch:
- Adds several ssh_config options:
  GSSAPIKeyExchange, GSSAPITrustDNS,
  GSSAPIClientIdentity, GSSAPIServerIdentity
  GSSAPIRenewalForcesRekey
- Optionally use an in-memory credentials cache api for security

My primary motivation for wanting the patch is the GSSAPIKeyExchange
and GSSAPITrustDNS features. My user ssh_config is shared across
several OSes, and it's a lot easier to manage if they all support the
same options.