Nagios plugin for checking remote SSH server parameters

List of parameters that can be checked:
- Blacklist of user authentication methods
- List of required user authentication methods

Example invocation:
  check_sshdparam --host localhost --timeout 5 \
    --uablacklist password,keyboard-interactive \
    --uawhitelist publickey

TODO:
- Host key fingerprint? Or just leave that to known_hosts.
- Host SSH string? Already done by check_ssh. Not in libssh2.

Built on libssh2 (as opposed to libssh) since it returns the
server's user auth list as a string. libssh sets flags, and
has a hardcoded (and brief) list of supported auth methods.