mdmon: improve switchroot handling.

mdmon: improve switchroot handling.

The change to get mdmon to re-exec itself from the switchroot
filesystem broken switchroot in various ways.  This fixes it.

If the switchroot path is not '/', mdmon will find the pid and
socket for the monitor, chroot to the new root, and exec mdmon
passing the pid in argv[2] and the socket in stdin.

If the switchroot path is actually a number, mdmon will not chroot,
but will kill that pid before taking over the array.

Signed-off-by: NeilBrown <neilb@suse.de>