mxqd: Use return() instead of exit()

This is just to avoid valgrind leak warnings. We have two variables with
cleanup attribute in the scope of main, which are not freed, when we
leave by exit() instead of running out of scope.

    ==10696== 21 bytes in 1 blocks are still reachable in loss record 1 of 3
    ==10696==    at 0x4C29AC6: malloc (vg_replace_malloc.c:299)
    ==10696==    by 0x5E19449: strdup (strdup.c:42)
    ==10696==    by 0x5E750DD: get_current_dir_name (getdirname.c:40)
    ==10696==    by 0x409F64: main (mxqd.c:2383)
    ==10696==
    ==10696== 45 bytes in 1 blocks are still reachable in loss record 2 of 3
    ==10696==    at 0x4C29AC6: malloc (vg_replace_malloc.c:299)
    ==10696==    by 0x40CFF9: mx_strvec_to_str (mx_util.c:1059)
    ==10696==    by 0x409F58: main (mxqd.c:2382)

mxqd.c

@@ -2553,6 +2553,5 @@ int main(int argc, char *argv[])
     mx_log_info("cu, mx.");
 
     mx_log_finish();
-
-    exit(0);
+    return(0);
 }