fcntl a little less

Setting O_CLOEXEC on closed file descriptors and getting E_BADF
should be faster than actually checking if an fd is open first.

src/fish.cpp

@@ -401,13 +401,14 @@ int main(int argc, char **argv) {
             res = reader_read(STDIN_FILENO, {});
         } else {
             char *file = *(argv + (my_optind++));
+#if defined(O_CLOEXEC)
+            int fd = open(file, O_RDONLY | O_CLOEXEC);
+#else
             int fd = open(file, O_RDONLY);
+#endif
             if (fd == -1) {
                 perror(file);
             } else {
-                // OK to not do this atomically since we cannot have gone multithreaded yet.
-                set_cloexec(fd);
-
                 wcstring_list_t list;
                 for (char **ptr = argv + my_optind; *ptr; ptr++) {
                     list.push_back(str2wcstring(*ptr));