Fixes memory leaks.

afCloseFile() uses free(), so we need to use malloc().

And specify whether generic tags are being scanned.

Meanwhile, GCC 5 fails to compile MPD due to incomplete C++14
implementation.

This way, IsFileNotFound() can detect status 404.

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

Allows callers to extract the NFS error code.