SRP code tidy.

Tidy up srp_Calc_k and SRP_Calc_u by making them a special case of
srp_Calc_xy which performs SHA1(PAD(x) | PAD(y)).

This addresses an OCAP Audit issue.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(cherry picked from commit 8f332ac9)