Cygwin: pthreads: merge pthread.cc into thread.cc

Cygwin: pthreads: merge pthread.cc into thread.cc

provide entire internal and external pthread API from inside the
same file.

While I dislike to have another even larger file, this is basically
cleaning up the source and grouping the external API into useful
chunks. Splitting the file cleanly is tricky due to usage of inline
methods is_good_object and verifyable_object_isvalid.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>