feature change: check system <hostname> command

[Rishi Dhupar]

Using monit 5.5, I have noticed the <hostname> param in a 'check
system' command is not optional.  The <hostname> provided seems to
override the $HOST variable name used in alerts, this does not seem
ideal.

I think it would be nice if the <hostname> param was optional and
monit could revert back to getting the FQDN name, this would allow me
to copy the same script files to many hosts and not make any changes.

Thanks for the great program.  Cheers.