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.

NEWS

@@ -4,6 +4,7 @@ ver 0.20 (not yet released)
   - "search"/"find" have a "window" parameter
   - report song duration with milliseconds precision
   - "sticker find" can match sticker values
+  - drop the "file:///" prefix for absolute file paths
 * tags
   - ape, ogg: drop support for non-standard tag "album artist"
     affected filetypes: vorbis, flac, opus & all files with ape2 tags