PathExists() should better do what CheckAccess() does, and
CheckAccess() doesn't do what its name implies.

Handles abstract sockets on Linux.

Fixes yet another breakage of creating the database+state file on
Windows.

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.

A few protocol feature have been added to the 0.20 branch, and thus we
should raise the protocol version in the server greeting.

The file handle is never reset to INVALID_HANDLE_VALUE, and thus the
destructor will assume the operation shall be cancelled and will
delete the temporary file.

This was a major breakage for saving the database file and the state
file.

Build a table of pre-existing tag types before adding new items.  The
old way would check HasType() each time, which would return true after
the first instance of that tag type had been added, preventing
duplicate tag types to be merged.

This broke duplicate tag types loaded from the state file, because
this code path uses TagBuilder::Complement().