handshake.
* assuan-client.c (read_from_server): Renamed to
(_assuan_read_from_server): this and made external.
* assuan-listen.c (assuan_set_hello_line): New.
(assuan_accept): Use a custom hello line is available.
* assuan-buffer.c (assuan_read_line): New.
(assuan_pending_line): New.
(_assuan_write_line): Renamed to ..
(assuan_write_line): this, made public and changed all callers.