Replace TLS_SIGALGS with SIGALG_LOOKUP

Since every supported signature algorithm is now an entry in the
SIGALG_LOOKUP table we can replace shared signature algortihms with
pointers to constant table entries.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2301)