mdmon: remove switch-root functionality.

mdmon: remove switch-root functionality.

Using switch-root and then creating files in /var/run once
that location is writable is racing as most distro clean out
/var/run shortly after it is mounted.
This can cause the .pid and .sock files to be deleted shortly
after they are created.

This option doesn't seem to be used at all any, and the alternative
of communicating the pid etc in some preserved directory and then
restarting mdmon once all is settled seems simpler.

So remove the code for supporting switchroot and update man page
accordingly.

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