Cleaned up tls1_prf and added comments.
Cleaned up tls1_prf and added comments.

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
Reviewed-by: Shane Lontis <452ce16516ceed26291fe7de3f8b53540d83864e@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8987)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl