ssl: TLSv1.2: proper default sign algo for RSA

Select {sha, rsa} signature algorithm when TLSv1.2 client
does not send signature_algorithms extension and negotiates
RSA key exchange.
See RFC 5246 section 7.4.1.4.1 for details.

Also this change fixes badarg when server tried to use
md5sha combination as public_key:sign algo.