Change SRP functions to use EVP_EncodeUpdate/EVP_DecodeUpdate functions

Previously they were using EVP_EncodeBlock/EVP_DecodeBlock. These are low
level functions that do not handle padding characters. This was causing
the SRP code to fail. One side effect of using EVP_EncodeUpdate is that
it inserts newlines which is not what we need in SRP so we add a flag to
avoid that.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5925)