fix optional session_id and clean_session args

The functions new and reinitialise takes an optional session id and
clean_session arg. Make those work properly.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
README: remove the session id from example code

It is no longer needed.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>