Included a summary of the checksrc.bat updates and combined two krb5
changes as they should have been implemented at the same time.

As this was introduced in the recent vauth changes and not a prior
release.

Closes #658

Closes #632

The CURLOPT_SSH_PUBLIC_KEYFILE option has been documented to handle
empty strings specially since curl-7_25_0-31-g05a443ad but the behavior
was unintentionally removed in curl-7_38_0-47-gfa7d04fe.

This commit restores the original behavior and clarifies it in the
documentation that NULL and "" have both the same meaning when passed
to CURLOPT_SSH_PUBLIC_KEYFILE.

Bug: http://curl.haxx.se/mail/lib-2016-01/0072.html

for all we know now, it might be called 7.46.1