Supports .mpdignore on NFS/SMB and others (closes #290).

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

Closes #169

Signed-off-by: FlashSystems <developer@flashsystems.de>

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

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

This reduces overhead for two reasons:

1. calls to Remove() are non-blocking

2. RunDeferred() may work on large chunks at a time, reducing the
   number of RunDeferred() calls

There's no point in passing a LightSong reference here; the callee is
interested only in the URI.

Use walk!=nullptr instead.

This is the final piece for full ID3/APE support in archives.

Don't open the ZIP file again and again for each song file.