Honour mandatory digest on private key in tls1_process_sigalgs()

If the private key says it can only support one specific digest, then
don't ask it to perform a different one.

Fixes: #7348

(cherry picked from commit 2d263a4a


 and reworked for 1.0.2)

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/7610)