If the FUSE daemon exits abnormally its mountpoint will not be unmounted.
To counter that the libfuse client may pass -o auto_unmount asking the library
to properly cleanup the mointpoint. This is achieved by starting a watchdog
process that first waits for the daemon process to finish and then unmointing
the FS if it is still there.
This change introduces the --auto-unmount option for mount_fusefs, an internal
hidden option inteded to be used only by libfuse.
The complementing libfuse part of changes is at https://github.com/libfuse/libfuse/compare/master...arrowd:libfuse:bsd-auto-unmount