osd: install admin socket commands after signals

This lets us tell by the presence of the admin socket commands whether
a signal will make us shut down cleanly.  See #5924.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit c5b5ce120a8ce9116be52874dbbcc39adec48b5c)