LibC: Templatize unique filename enumeration for mkstemp() et al

This allows us to implement mkstemp() with open() directly, instead of
first lstat()'ing, and then open()'ing the filename.

Also implement tmpfile() in terms of mkstemp() instead of mktemp().

Userland/Libraries/LibC/stdio.cpp

@@ -1205,16 +1205,11 @@ void funlockfile([[maybe_unused]] FILE* filehandle)
 FILE* tmpfile()
 {
     char tmp_path[] = "/tmp/XXXXXX";
-    if (__generate_unique_filename(tmp_path) < 0)
-        return nullptr;
-
-    int fd = open(tmp_path, O_CREAT | O_EXCL | O_RDWR, S_IWUSR | S_IRUSR);
+    int fd = mkstemp(tmp_path);
     if (fd < 0)
         return nullptr;
-
     // FIXME: instead of using this hack, implement with O_TMPFILE or similar
     unlink(tmp_path);
-
     return fdopen(fd, "rw");
 }
 }