Non-static functions replaces the GROUP_COMBINE etc methods.

(abstract_group): Deleted class and
methods.
(zn_group): Declare class here, and plain functions for using it.
Replaced all uses of struct abstract_group with const struct
zn_group.
Use const for all dh_method pointers.

Rev: src/client_keyexchange.c:1.68
Rev: src/client_keyexchange.h:1.19
Rev: src/dh_exchange.c:1.19
Rev: src/server_keyexchange.c:1.58
Rev: src/server_keyexchange.h:1.13
Rev: src/srp-gen.c:1.22
Rev: src/srp.h:1.8
Rev: src/srp_exchange.c:1.21