Reduce some unnecessary overhead.

Replaces StatFile(), with a portable data object.

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

The old struct config_param remains only for top-level string options.

Return false on error, not nullptr.

New in clang 3.6.

Fixes -Winconsistent-missing-override (clang 3.6).

Simplify the definition of many build options.

Eliminate one implicit std::string copy.

The Release Track Id uniquely identifies a recording on a release - that
is, even if a recording appears twice on a release (meaning that the
combination of recording and release id are not enough to figure out
which one it is), the release track id will allow differentiating the two.

The tag names are taken from
https://musicbrainz.org/doc/MusicBrainz_Picard/Tags/Mapping

Workaround for assertion failure in PrintUniqueTag().

Use milliseconds precision to reduce rounding errors.

The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

Fixes potential crash.

Check for 0 in "device" and "inode" instead.

Remove the C list_head library and use type-safe C++ instead.