Use closefrom() to close open file descriptors

Replace the naive close() loop in js_os_exec with closefrom().

On my system RLIMIT_NOFILE is set to 1 million and the delay from the
loop gets noticeable when I spawn many processes.

Fixes: #711

quickjs-libc.c

@@ -3123,18 +3123,20 @@ static JSValue js_os_exec(JSContext *ctx, JSValue this_val,
     }
     if (pid == 0) {
         /* child */
-        int fd_max = sysconf(_SC_OPEN_MAX);
-
         /* remap the stdin/stdout/stderr handles if necessary */
         for(i = 0; i < 3; i++) {
             if (std_fds[i] != i) {
                 if (dup2(std_fds[i], i) < 0)
                     _exit(127);
             }
         }
-
+#if defined(__APPLE__)
+        int fd_max = sysconf(_SC_OPEN_MAX);
         for(i = 3; i < fd_max; i++)
             close(i);
+#else
+        closefrom(3);
+#endif
         if (cwd) {
             if (chdir(cwd) < 0)
                 _exit(127);