Use thread-local storage of GCC 4.x

Use thread-local storage of GCC 4.x

ucw/threads.c can now work in three different modes:

   o  non-threaded
   o  threaded with context in pthread per-thread storage
   o  threaded with context stored as __thread

I had to make a backward-incompatible change: ucwlib_context->thread_id
is no longer accessible directly (because I do not know of any way how
to initialize it in the __thread mode) and a wrapper function should
be used.