Change our view of the Diffie--Hellman closure protocol.

Rename `len' to `secret_len' and `ceil_len' to `shared_len', and label
these as the required private secret length and generated shared-secret
length respectively.  There's no functional change: that's what these
variables were being used for anyway.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>