[Libguestfs] [PATCH nbdkit INCOMPLETE] New filter: exitwhen: exit gracefully when an event occurs.

[Richard W.M. Jones]

This incomplete patch adds a new filter allowing more control over
when nbdkit exits.  You can now get nbdkit to exit gracefully on
certain events, such as a file being created, a pipe held open by
another process going away, or when another PID exits.  There is also
a script option to allow for completely custom events.

It is untested at the moment, I'm posting it to get feedback on the
idea.  The manual page is a complete description of the proposed
feature.

To do:
 - implement the polling thread
 - add tests

Rich.