Prepare for a ICU-based backend without GLib.

Don't expose pointer that requires the caller to invoke g_free(),
because that's GLib-only.

Don't abort the process, let the caller decide instead.

MPD_PATH_MAX_UTF8 is only used by GLib-specific code currently.

The upper-case name seems to be the canonical one.

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.

Implement a fast path for UTF-8 which leaves fs_charset empty, and
don't assign a value to fs_charset if there's no configuration.

Move code from map_fs_to_utf8().

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

Migrate the remaining callers to FatalError().

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.