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.

Replaces GLib's g_str_has_prefix().

If fs_charset is empty, i.e. we're using the default "utf-8",
GetFSCharset() should return exactly that instead of an empty
std::string.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Move to TagSettings.c and use C99 initializers.