Eliminate one unnecessary dependency for debug programs which don't
need the InputStream API.

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

Move code from config_global_finish().

Move code from Path::RelativeFS() and make it generic.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

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.

It is illegal to use it after going out of scope.

WinAPI explicitly declares filesystem encoding.
It can be determined by GetACP().
Use that instead of Glib routine that always "detects" UTF-8 on Win32,
which is incorrect for MPD case.