Follow up e0c8e01

Move skill_get_cooldown_ into pc_get_skillcooldown
and add a short src_doc description to promote his usage
(and avoid duplicate code)
Fix few cppcheck issues (variable scope optimisation mostly)

src/map/script.c

@@ -9151,7 +9151,7 @@ BUILDIN_FUNC(monster)
 
 	for(i=0; i<amount; i++){ //not optimised
 		int mobid = mob_once_spawn(sd, m, x, y, str, class_, 1, event, size, ai);
-		mapreg_setreg(reference_uid(add_str("$@mobid"), i),mobid);
+		if(mobid) mapreg_setreg(reference_uid(add_str("$@mobid"), i),mobid);
 	}
 	return SCRIPT_CMD_SUCCESS;
 }