Add a special case for the DH group size for 3des-cbc, which has an
effective strength much lower than the key size.  This causes problems
with some cryptlib implementations, which don't support group sizes larger
than 4k but also don't use the largest group size it does support as
specified in the RFC.  Based on a patch from Petr Lautrbach at Redhat,
reduced by me with input from Markus.  ok djm@ markus@
import digest API - not yet wired in