Make pg_ctl use SIGINT as a default shutdown signal.

The commit 0badb069 changed the default shutdown mode from smart to fast,
but forgot to change the default shutdown signal from SIGTERM to SIGINT.

src/bin/pg_ctl/pg_ctl.c

@@ -80,7 +80,7 @@ static bool wait_set = false;
 static int	wait_seconds = DEFAULT_WAIT;
 static bool silent_mode = false;
 static ShutdownMode shutdown_mode = FAST_MODE;
-static int	sig = SIGTERM;		/* default */
+static int	sig = SIGINT;		/* default */
 static CtlCommand ctl_command = NO_COMMAND;
 static char *pg_data = NULL;
 static char *pg_config = NULL;