site: No longer track key validity ourselves
We used to have a variable st->current_valid, which we would always
set to True when assigning a key to current_transform, and to False
when calling delkey (or on initialisation).
Now that we have transform->valid(), this is no longer needed.
We do introduce a trivial helper function current_valid(st) to help us
call it, so the places where st->current_valid was checked don't get
too ugly.
There is no intentional change to the externally-visible behaviour.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>