driver: Advance the eRoT's own floor when the update agent asks

The counterpart to settle_self_update's hold: commit_self_svn_floor reads
the SVN the confirmed session recorded, advances the eRoT's floor, and
closes the session. The caller runs it when the FD reports
SvnCommitPending, and answers the update agent with what comes back: done
on Ok, refused on Err. The request's own SVN is not read; the session is
what says which SVN the floor may take.

Refused unless the session is confirmed and still open, so a request with
nothing behind it cannot move the floor: no image has proven itself at
that SVN.

Safe to repeat across the crash window between the advance and the close.
advance is a no-op at or below the floor, so a request repeated after a
crash finishes the close. A repeat after the session is closed is refused,
so an answer that never reached the update agent leaves its retry refused
with the floor already where it asked. A floor that cannot take the SVN
leaves the session open, so the update agent's next request tries again
instead of losing the advance.

Takes the session and the floor directly, as settle_self_update does.
Neither is board wiring yet.

Assisted-by: Claude