site: Check transform errors; factor out transform handling

Make sure we always check the error return from transform->forwards
and ->backwards.  Otherwise logic errors in the site state machine
might result in us sending out packets with unencrypted insider
plaintext, or the like, due to the transform being unkeyed when we try
to use it.

Factor some repeated idioms for transform handling into a set of new
functions.  This will make the next patch much easier.

We arrange to pass dispose_transform a pointer to the actual state
variable where the transform is kept; this means that it can be
changed to update that pointer.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>