* src/assuan-defs.h (struct assuan_context_s): Have w32_error field.
(_assuan_sock_wsa2errno): Supply CTX argument.
* src/assuan-socket.c (_assuan_sock_wsa2errno): Put ERR to the field
in CTX argument.
(_assuan_sock_get_flag): Support "w32_error" string.
(_assuan_sock_accept, _assuan_sock_bind): Follow the change.
* src/assuan-uds.c (uds_reader, uds_writer): Likewise.
* src/system-w32.c (__assuan_close, __assuan_socket): Likewise.
(__assuan_connect): Likewise.