Add -reincarnate flag to FS

When FS is given the -reincarnate flag, FS will be automatically
restarted after any uncontrolled exit.
Handle signals better with -reincarnate

If we receive SIGTERM or SIGILL we should propagate the signal to the
child FS process.  As FS normally handles these signals, we need to
restore the handler before we refork.

FS may also add a handler for SIGCHLD; we need the default action
here instead for the parent.
Kill FS if -reincarnate parent dies unexpectedly

This only works on Linux.