Improvement of Linux supports and  NFS handling

It improve nfs handling in Linux hosts.

It changes linux plugin  to use ```/usr/sbin/exportfs -r``` standard command  instead of restarting nfs daemon.
To realize it, it checks whether nfs daemon running or not. If not running, starting daemon as similar way as before.

Also it add a support for Slackware and its derivative, that  close #2075