command/{Queue,File}Commands: drop "file:///" prefix for absolute paths

Requiring this prefix makes the client's intention very clear, but it
was too hard to understand why this prefix was needed.  Initially, my
intention was to differentiate from broken clients which prefix relate
URIs with a slash; once MPD allowed that.  In the past few years
however, MPD has disallowed that, and there was no significant
breakage (except for the "add /" special case which some clients
apparently still do).  So I figure it's about time to define that an
URI that begins with a slash points to an arbitrary file on the file
system.

doc/protocol.xml

@@ -1772,7 +1772,7 @@ OK
             <para>
               Clients that are connected via UNIX domain socket may
               use this command to read the tags of an arbitrary local
-              file (URI beginning with "file:///").
+              file (URI is an absolute path).
             </para>
           </listitem>
         </varlistentry>
@@ -1787,8 +1787,7 @@ OK
             <para>
               Read "comments" (i.e. key-value pairs) from the file
               specified by "URI".  This "URI" can be a path relative
-              to the music directory or a URL in the form
-              "file:///foo/bar.ogg".
+              to the music directory or an absolute path.
             </para>
             <para>
               This command may be used to list metadata of remote