* Simultaneous execution limits

It is possible to limit number of command instances that are allowed
to run simultaneosly for a particular watcher.  This is done using
the 'max-instances' statement in 'watcher' section.

* Restore the nowait default

Unless given the 'nowait' option explicitly, watchers waited for the
handler to terminate.  It is now fixed and 'nowait' is the default, as
described in the documentation.

* Fix bug in generic to system event translation

* Fix sentinel code

In some cases setting the sentinel effectively removed the original
watcher.  That happened if the full file name of the original watcher
and its directory part produced the same hash.