The underlying library has been obsolete for many years.

Hide the definition from C code, to prepare the transition to C++.

- DirectoryReader is extracted to separate header
- FileSystem.cxx/FileSystem.hxx/DirectoryReader.hxx moved to fs/ subdir
- Functions return true on success, instead of 0 (where applicable)
- ReadLink return result instead of out parameter
- UnlinkFile is renamed to RemoveFile
- CheckExists/CheckIsRegular/CheckIsDirectory are renamed
  to PathExists/FileExists/DirectoryExists

This file was used previously to force C++ linkage.
It's no longer required because most of the code is C++ anyway.

Use CRITICAL_SECTION and CONDITION_VARIABLE.  This requires Windows
Vista or newer.  It fixes problems with GLib threading objects that
were implicitly created by static constructors before g_thread_init().