mp_threads: rename threads for consistent naming across all of them

I'd like some names to be more descriptive, but to work with 15 chars
limit we have to make some sacrifice.

Also because of the limit, remove the `mpv/` prefix and prioritize
actuall thread name.

common/msg.c

@@ -541,7 +541,7 @@ static void *log_file_thread(void *p)
 {
     struct mp_log_root *root = p;
 
-    mpthread_set_name("log-file");
+    mpthread_set_name("log");
 
     pthread_mutex_lock(&root->log_file_lock);