Improving mk_filehandler to support more options

Hi,

I have a need to monitor open file handles for separate users, which have their own limits.
There’s a plugin for it already made (https://github.com/HeinleinSupport/check_mk_extensions/tree/master/filehandles) but it would be great to have this officially supported.
Currently, the mk_filehandler doesn’t really do much. Just checks the global limits.

Cheers!