update: apply .mpdignore matches to subdirectories

Wildcard matches are directly applied to all filenames in
subdirectories without any attempt at matching relative paths.

This change is based on the following feature request:

  http://bugs.musicpd.org/view.php?id=3729

doc/user.xml

@@ -1077,6 +1077,8 @@ database {
         To exclude a file from the update, create a file called
         <filename>.mpdignore</filename> in its parent directory.  Each
         line of that file may contain a list of shell wildcards.
+        Matching files in the current directory and all subdirectories
+        are excluded.
       </para>
     </section>