OpenSSH: Keychain support

* Adds an option '--with-keychain-support' that adds native OS X
  Keychain capabilty and LaunchDaemon capabilities back to ssh-agent.
* Added caveat.

Closes #187.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>
OpenSSH: Disable header/library version check.

* Sometimes Apple ships security updates for libraries while header
  files are intact, which leads to inconsistent versions between them,
  but it should be harmless. Disable such check in openssh to workaround
  this limitation.