w32: Change the way the I/O threads are cleaned up.
* src/w32-io.c (reader_context_s, create_reader) (writer_context_s, create_writer): Rename STOPPED to CLOSE_EV. (reader, writer): Remove setting of STOPPED. Wait for CLOSE_EV and then release the context. (destroy_reader, destroy_writer): Do not wait but set the CLOSE_EV. (kill_reader, kill_writer): Remove. (_gpgme_io_close): Add code from kill_reader and kill_writer.
The old code was prone to deadlocks which were actually exhibited at
Kleopatra startup. The new code is much more straightforward and
easier to understand. The reason for the complex old code was
probably due to our former idea to allow re-use of the I/O threads.
However we have long given up on this.