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

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.