eventscripts: Add new option $CTDB_MONITOR_NFS_THREAD_COUNT

eventscripts: Add new option $CTDB_MONITOR_NFS_THREAD_COUNT

Consider the following example:

1. There are 256 nfsd threads configured.
2. 200 threads are "stuck" in system calls, perhaps waiting for the
   underlying filesystem when an attempt is made to restart NFS.
3. 56 threads exit when NFS is stopped.
4. 56 new threads are started when NFS is started.
5. 200 "stuck" threads exit leaving only 56 threads running.

Setting this option to "yes" makes the 60.nfs monitor event look for
this situation and try to correct it.

Signed-off-by: Martin Schwenke <martin@meltin.net>