vtls: move md5sum into the Curl_ssl struct

The MD5 summing is also an SSL backend-specific function. So let's
include it, offering the previous fall-back code as a separate function
now: Curl_none_md5sum(). To allow for that, the signature had to be
changed so that an error could be returned from the implementation
(Curl_none_md5sum() can run out of memory).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>