For improved portability of the net library to projects without
autoconf.

Not used anymore.

There is not much use in GLib: on Windows, we have a native API for
string collation, and the rest uses either libicu or the standard C
library calls.

We don't need this anymore: Win32 doesn't use this library at all, and
everything else has either iconv() or libicu.

default_value defaults to nullptr.

Since we are removing GLib, this call is useless.

I suppose there is no non-WIN32 operating system that doesn't have
fnmatch().  So let's remove the GLib implementation.

Drop support for misencoded tags.  People should not be using ID3v1
anyway.

The default is always UTF-8.

Always write UTF-8 to the log file.