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.

See commit be137a19

Since 7d353bbe, _GNU_SOURCE is always
defined, which implies __USE_MINGW_ANSI_STDIO and thus switches to
the mingw implementations of the printf() family.  That's
standards-compliant, unlike Microsoft's CRT implementations.

This code was added in 21851c06 but
looks completely broken:

- the status code is "206 OK" but "206" would be "Partial Content"

- the "Content-Length" header has a bogus value

- the "Content-RangeX" parameter has different bogus values (why
  "Content-RangeX" anyway and not "Content-Range"?)

Apart from that, there are strange undocumented non-standard headers
which are probably there to work around bugs/expectations in one
broken proprietary client product.  But these days, MPD doesn't bend
over to support broken clients.  So let's kill this code.

Closes #304

This enforces the "not nullptr" rule.

Prepare to make LightSong non-assignable.

AudioOutputControl::WaitForCommand() doesn't need this check.

Prepare for adding another Cond attribute.

Since commit e81b0896, this is the
same code as in ReplacePipe().