res_musiconhold.c: Use ast_file_read_dir to scan MoH directory

Two changes of note in this patch:

* Use ast_file_read_dir instead of opendir/readdir/closedir

* If the files list should be sorted, do that at the end rather than as
  we go which improves performance for large lists

Change-Id: Ic7e9c913c0f85754c99c74c9cf6dd3514b1b941f