Add function tls_choose_sigalg().

New function tls_choose_sigalg(). This is a signature algorithm version
of ssl3_choose_cipher(): it picks and sets the appropriate signature
algorithm and certificate based on shared signature algorithms.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2339)