Add pid file support

[zachomedia]

This PR adds support for writing a PID file. This lets us clean up our Kubernetes flexvol mounts (https://github.com/StatCan/s3-fuse-flex-volume) since we can track the pid of the goofys instance we launch.

[kahing]

Thanks @zachomedia . Could you put this next to -f under misc options. Also explain that this doesn't work with -f. Maybe something like:

Write the background process id to this file. Does nothing if -f is used`.

[zachomedia]

@kahing Thanks, I've made the requested change.