This doesn't work because IterableSplitString() returns its elements
by value.

Fixes clang warning:

 loop variable 'i' is always a copy because the range of type 'IterableSplitString' (aka 'BasicIterableSplitString<char>') does not return a reference [-Werror,-Wrange-loop-analysis]

Eliminates yet another string copy.

"De-facto" field mappings are available at http://wiki.hydrogenaud.io/index.php?title=Tag_Mapping.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Prepare for APE tag support on userspace NFS/SMB/CIFS mounts.

This was used by proprietary software.  MPD adopted it a few years
ago, which turns out to be a mistake, because it now creates problems
for some MPD users (http://bugs.musicpd.org/view.php?id=4168).

Reduce header dependencies.

WavPack tags are always APEv2, by definition.  Reuse the tag_table
from tag_ape.c, instead of rolling our own.

Ignore APE tags that have no usable tags, and use the ID3 tag instead.
This is useful when the APE tag only contains replay gain, and the
real tags are stored as ID3.  This implements feature request Mantis
#0003521.

Use _scan() instead, to have more control.

The new method pair() receives an arbitrary name/value pair.  Support
for this is being added to a few decoder plugins.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

The struct is smaller because it is sparse.  Its traversal is also
more efficient.