Move 'shared_sigalgs' from cert_st to ssl_st

It was only ever in cert_st because ssl_st was a public structure
and could not be modified without breaking the API.  However, both
structures are now opaque, and thus we can freely change their layout
without breaking applications.  In this case, keeping the shared
sigalgs in the SSL object prevents complications wherein they would
inadvertently get cleared during SSL_set_SSL_CTX() (e.g., as run
during a cert_cb).

Fixes #9099

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9157)

(cherry picked from commit 29948ac80c1388cfeb0bd64539ac1fa6e0bb8990)