Add statistics from /proc/self/mountstats for nfs clients (like iostat -n)

Hello,

On a nfs client, run "iostat -n" and  have a look on /proc/self/mountstats...

This patch parses /proc/self/mountstats and get statistics about all NFS mountpoints.

Note : patch against release 5.1.1 is in perfwatcher/collectd-pw in branch patch-nfs_stats_with_network_plugin/5.1.1 if you are also interested in it.

Regards,
Yves