mds: Fix respawn (add path resolution)

Previously assumed that ceph-mds executable was in
PWD - now use /proc/self/exe to find the
executable whereever it may be.  Leave in old version
as a fallback for non-linux environments.

Also add a 'respawn' command so that it's easy to test
respawn with `ceph mds tell <id> respawn`

Fixes: #7966