Disable the undocumented xmlvalidate() function, which was unintentionally

Disable the undocumented xmlvalidate() function, which was unintentionally
left in the code though it was not meant to be provided.  It represents a
security hole because unprivileged users could use it to look at (at least the
first line of) any file readable by the backend.  Fortunately, this is only
possible if the backend was built with XML support, so the damage is at least
mitigated; and 8.3 probably hasn't propagated into any security-critical uses
yet anyway.  Per report from Sergey Burladyan.