Use -nofork for bosserver in openafs-fileserver.service

Use -nofork for bosserver in openafs-fileserver.service

So that we can use Type=simple.
This fixes an issue where systemd would stop tracking us after
a rebozo event (like weekly restarts or bos restart -bosserver),
since after re-exec() we also re-daemon(), and the parent process
(which is the one systemd is tracking) exits cleanly.  That's taken
as a signal to shut things down with the ExecStop command of
bos shutdown, which works as intended and leaves things stopped.

This slightly increases the window between when systemd thinks
we are started and ready to serve traffic and the time when we
are actually ready to serve traffic (now systemd thinks we're ready
at process start, versus when the parent exits), but that difference
is just argument parsing, loading up the bnode types, creating dirs
and switching to them.  It doesn't include starting up Rx or the
child processes for the configured bnodes.

Change-Id: I8d8665e529ecd8f679b5f2c697991333f6d1a7a6