Use SIGNAL_ARGS consistently to declare signal handlers.

Various bits of code were declaring signal handlers manually,
using "int signum" or variants of that.  We evidently have no
platforms where that's actually wrong, but let's use our
SIGNAL_ARGS macro everywhere anyway.  If nothing else, it's
good for finding signal handlers easily.

No need for back-patch, since this is just cosmetic AFAICS.

Discussion: https://postgr.es/m/2684964.1663167995@sss.pgh.pa.us

src/backend/storage/ipc/pmsignal.c

@@ -88,7 +88,7 @@ NON_EXEC_STATIC volatile PMSignalData *PMSignalState = NULL;
 volatile sig_atomic_t postmaster_possibly_dead = false;
 
 static void
-postmaster_death_handler(int signo)
+postmaster_death_handler(SIGNAL_ARGS)
 {
 	postmaster_possibly_dead = true;
 }