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
Merge pull request #1671 from ceph/wip-7699

mds: Fix respawn (add path resolution)

Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>