Avoid duplicate AllocatedPath::FromUTF8() invocations in two callers.

Wrapper for protocol/ArgParser.cxx.

Replaces StatFile(), with a portable data object.

Previously, MPD printed the time stamp without printing the file
name.  That made no sense.

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.

Mapped to "=" in the MPD protocol.  This is the first operator,
initially supporting value matches in the MPD protocol.