SSH: fix CURLOPT_SSH_HOST_PUBLIC_KEY_MD5

When a 32 digit hex key is given as a hostkey md5 checksum, the code
would still run it against the knownhost check and not properly
acknowledge that the md5 should then be the sole guide for.

The verbose output now includes the evaluated MD5 hostkey checksum.

Some related source code comments were also updated.

Bug: http://curl.haxx.se/bug/view.cgi?id=3451592
Reported by: Reza Arbab