Rework parse_notification() to use the ibuf API for everything.

Rework parse_notification() to use the ibuf API for everything.

While there fix the RFC5492 handling of ERR_OPEN_CAPA (the current code
has the logic inversed). ERR_OPEN_CAPA is there to signal that a needed
capability is missing in our OPEN message. Just add the handling of
ERR_OPEN_CAPA to log_notification().

Also rework the handling of the shutdown reason and move the printing
into log_notification().

OK tb@