Add support for fetching check_mk data from non unix systems

The unix_agent.inc.php file explicitly looks for the os_group to be "unix", and thus excludes the possibility of collecting data from any other OS which is running the check_mk program, or equivalent.
Since the check_mk program exists for Windows, and the concept of just returning a bunch of text is pretty simple, it would be nice to add allow other OS's to use this agent to put data in Observium.

Attached is a patch to add this capability as a configuration option on a per os basis.