Make Watchdog Signal Configurable

Allows configuring the watchdog signal (with a default of SIGABRT).
This allows an alternative to SIGABRT when coredumps are not desirable.

Appropriate references to SIGABRT or aborting were renamed to reflect
more liberal watchdog signals.

Closes #8658

src/shared/bus-unit-util.c

@@ -1222,7 +1222,7 @@ static int bus_append_kill_property(sd_bus_message *m, const char *field, const
 
                 return bus_append_parse_boolean(m, field, eq);
 
-        if (STR_IN_SET(field, "KillSignal", "FinalKillSignal"))
+        if (STR_IN_SET(field, "KillSignal", "FinalKillSignal", "WatchdogSignal"))
 
                 return bus_append_signal_from_string(m, field, eq);