openssh: restore keychain and gsskex patches

This restores the keychain and gsskex patches.

The ssh-add command showed the "-M" and "-m" options but wouldn't allow
you to specify them since they weren't added to the getopt(...) call.
Point the patch URL to a corrected gist.

Closes #237.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>