postmaster: Improve logging of signals sent by postmaster

postmaster: Improve logging of signals sent by postmaster

Previously many, in some cases important, signals we never logged. In other
cases the signal name was only included numerically.

As part of this, change the debug log level the signal is logged at to DEBUG3,
previously some where DEBUG2, some DEBUG4.

Also move from direct use of kill() to signal the av launcher to
signal_child(). There doesn't seem to be a reason for directly using kill().

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/kgng5nrvnlv335evmsuvpnh354rw7qyazl73kdysev2cr2v5zu@m3cfzxicm5kp