check_ssh: check protocol

It would be useful to be able to detect the protocols supported by the remote
ssh server to locate any using the insecure ssh v1 protocol. This patch
attempts to match against the protocol string in the ssh response.

Example:

check_ssh -H my.host.com -P 2.0
--
Just turning attached patch of github issue #780 into a push request.
(Closes #780)