Use _exit in CGI. Close cesanta/mongoose#779

PUBLISHED_FROM=bfe62d3824ee3a6e66b7d1a57ee7bad3b726649d

mongoose.c

@@ -7873,7 +7873,7 @@ static int mg_start_process(const char *interp, const char *cmd,
              interp == NULL ? "" : interp, interp == NULL ? "" : " ", cmd,
              strerror(errno));
     send(1, buf, strlen(buf), 0);
-    exit(EXIT_FAILURE); /* exec call failed */
+    _exit(EXIT_FAILURE); /* exec call failed */
   }
 
   return (pid != 0);