dh: Add ssh_dh_debug_crypto()

We should call it where we have access to the crypto structure.

Pair-Programmed-With: Jakub Jelen <jjelen@redhat.com>
Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>