Fixes memory leaks.

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

And specify whether generic tags are being scanned.

Also test the newest available GCC version.

After commit a72d1200, the Travis-CI
build fails, because we have been using GCC 5 there...  so let's
switch to GCC 6 to test the oldest possible GCC version.

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.