Commit b3a45833 added a LocateUri()
call to several playlist commands, which applied InputPlugin URI
scheme verification to playlist URIs.  This broke the SoundCloud
playlist plugin which uses "soundcloud://" URIs for which no input
plugin exists.

This commit allows the caller to specify the kind of plugin which
shall be used to verify the URI.  Right now, only "input" is
implemented; "storage" uses the "input" verification for now; and
"playlist" has no verification at all (for now).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/528

Must lock the mutex before calling any of the unprotected InputStream methods.

Closes #462

Modeled after std::filesystem::operator/() from C++17.

And specify whether generic tags are being scanned.

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

read_stream_art uses PRIu64 unconditionally with the Format
method of a Respone instance to output a size_t typed value.
If size_t is 32bit the output is garbeled. This patch uses
offset_type and PRIoffset to make sure the format string
and the type of the output value always match.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Add API documentation
Support 64 bit offsets
Use InputStream for all reads

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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.

Lists files and directories.  Supports storage plugins.