signal_handler: Implementing specific messages per context

signal_handler: Implementing specific messages per context

As per bug #23320, regarding the signal we receive, it could be interesting providing a specific message.

If we get a SI_USER we almost print the actual message except we don't print the PID which have no meaning when set to 0.

In all other cases, we do print the full structure to help debuggers understanding the received signal.

This patch is offering a better way to get specific messages based on various si_code values.
It could be expanded later based on debugging feedbacks.

Fixes: http://tracker.ceph.com/issues/23320
Signed-off-by: Erwan Velu <erwan@redhat.com>