w32: Better protect the IO-system's fd_table

* src/w32-io.c (fd_table_lock): New.
(new_fd): Lock allocation of a new slot.
(release_fd): Lock deallocation of a slot.

Note that we lock only the allocation but not the sanitiy checks we do
further down in the code.

• Signed-off-by: Werner Koch <wk@gnupg.org>