[krbdev.mit.edu #7790] PoC to fix cross realm S4U2Self

[Greg Hudson via RT]

Microsoft's answer confirmed that an enterprise principal is the most 
general way for a server to identify itself when making an S4U2Self 
request to another realm's KDC, and expressed an intent to update their 
documentation (but they don't seem to have done so yet).

I thought I had previously heard that you had a cleaned-up version of 
this patch.  If so, you could make a pull request on github.com/krb5 or 
send it here.  If all you have so far is the PoC patch, we will find 
time to clean it up and integrate it some time during the 1.14 
development cycle.