mgr: handle SIGTERM/SIGINT in standby mgr to avoid CEPHADM_FAILED_DAEMON

MgrStandby only registered SIGHUP, leaving SIGTERM/SIGINT at OS defaults.
When a standby is stopped via `ceph orch daemon stop`, SIGTERM terminates
it with exit code 143, causing systemd to mark the unit as failed instead
of stopped, which triggers CEPHADM_FAILED_DAEMON.

Mirror the handler already used in Mgr::init() for the active mgr.

Fixes: https://tracker.ceph.com/issues/77116
Signed-off-by: Nitzan Mordechai <nmordech@ibm.com>