[krbdev.mit.edu #3265] Compile on Debian GNU (Hurd)

[Public Submitter via RT]

The following patch has been applied to the Debian package to support
compilation on the Hurd.  The main change is that MAXPATHLEN and
MAXHOSTNAMELEN are not defined on the Hurd and therefore cannot be used
without defining them first.

There may be a better approach than defining them in every file that
uses them, which is what this patch does.

The Hurd also requires declaring a few additional internal functions as
weak for proper pthreads support.  It's possible that this may be fixed
later; applications really shouldn't have to know about this.