Rather than support arbitrarily many handshake hashes in the general
case (which the PRF logic assumes is capped at two), special-case the
MD5/SHA1 two-hash combination and otherwise maintain a single rolling
hash.

Change-Id: Ide9475565b158f6839bb10b8b22f324f89399f92
Reviewed-on: https://boringssl-review.googlesource.com/5618
Reviewed-by: Adam Langley <agl@google.com>