Extend ServerKeyExchange parsing to work with a signature

Previously SKE in TLSProxy only knew about one anonymous ciphersuite so
there was never a signature. Extend that to include a ciphersuite that is
not anonymous. This also fixes a bug where the existing SKE processing was
checking against the wrong anon ciphersuite value. This has a knock on
impact on the sslskewith0p test. The bug meant the test was working...but
entirely by accident!

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