Convert Sigalgs processing to use ints

In TLSv1.2 an individual sig alg is represented by 1 byte for the hash
and 1 byte for the signature. In TLSv1.3 each sig alg is represented by
two bytes, where the two bytes together represent a single hash and
signature combination. This converts the internal representation of sigalgs
to use a single int for the pair, rather than a pair of bytes.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2157)