Rename SSL[_CTX]_add1_CA_list -> SSL[_CTX]_add1_to_CA_list

They add a single item, so the names give a false impression of what
they do, making them hard to remember.  Better to give them a somewhat
better name.

Fixes #6930

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6931)