Add open_files capability to ejabberd_resources_

It appears that erlang port is not bound to an OS file very often.
Thus "ulimit -n" is moved to a separate graph and lsof/fstat is used as
a backend to count the number of files open.
Merge pull request #403 from dotdoom/master-ejabberd_resources_-file-descriptors

Add open_files capability to ejabberd_resources_