MINOR: startup: add O_CLOEXEC flag to open /dev/null

As master process performs execvp() syscall to handle USR2 and HUP signals in
mworker_reexec(), let's add O_CLOEXEC flag, when we open '/dev/null' in order
to avoid fd leak.

This a preparation step to refactor master-worker logic. See more details in
the next commits.

src/haproxy.c

@@ -3704,7 +3704,7 @@ int main(int argc, char **argv)
 		}
 
 		if (!(global.mode & MODE_QUIET) || (global.mode & MODE_VERBOSE)) {
-			devnullfd = open("/dev/null", O_RDWR, 0);
+			devnullfd = open("/dev/null", (O_RDWR | O_CLOEXEC), 0);
 			if (devnullfd < 0) {
 				ha_alert("Cannot open /dev/null\n");
 				exit(EXIT_FAILURE);