Check that MyProcPid == getpid() in backend signal handlers.

Check that MyProcPid == getpid() in backend signal handlers.

In commit 97550c0711, we added a similar check to the SIGTERM
handler for the startup process.  This commit adds this check to
backend signal handlers installed with pqsignal().  This is done by
using a wrapper function that performs the check before calling the
actual handler.

The hope is that this will offer more general protection against
child processes of Postgres backends inadvertently modifying shared
memory due to inherited signal handlers.  Another potential
follow-up improvement is to use this wrapper handler function to
restore errno instead of relying on each individual handler
function to do so.

This commit makes the changes in commit 97550c0711 obsolete but
leaves reverting it for a follow-up commit.

Reviewed-by: Andres Freund, Noah Misch
Discussion: https://postgr.es/m/20231121212008.GA3742740%40nathanxps13