stream: minor cleanups

Fix return types and return values to make them more consistent. Some
reformatting and making code more concise.

In stream_reconnect(), avoid the additional mp_cancel_test() call by
moving the "connection lost" message below the mp_cancel_wait() call,
which effectively leads to the same behavior when the stream was already
canceled. (The goal is not to show the message in this case.)

Merge stream_seek_long() into stream_seek(). It was the only caller.

Always clear the eof flag on seeks.

Reduce access to stream internals in cache.c and stream_lavf.c.

stream/cache.c

@@ -581,9 +581,8 @@ static int cache_control(stream_t *cache, int cmd, void *arg)
     }
     r = s->control_res;
     if (s->control_flush) {
+        stream_drop_buffers(cache);
         cache->pos = s->read_filepos;
-        cache->eof = 0;
-        cache->buf_pos = cache->buf_len = 0;
     }
 
 done: