* src/assuan-socket.c (cygwin_fdtable, cygwin_fdtable_cs): New.
(is_cygwin_fd, insert_cygwin_fd, delete_cygwin_fd): New.
(assuan_sock_init) [W32]: Init the CS.
(assuan_sock_deinit) [W32]: Deinit the CS.
(read_port_and_nonce): Add arg cygwin and detect Cygwin socket files.
(_assuan_sock_set_flag): Add "cygwin" flag.
(_assuan_sock_get_flag): Ditto.
(do_readn, do_writen): New.
(_assuan_sock_bind): Create a Cygwin socket file depending on a socket
flag.
(_assuan_sock_connect): Handle the cygwin socket protocol.
(_assuan_sock_check_nonce): Ditto.