[krbdev.mit.edu #7071] PKINIT trusted_ca encoding issues

[Greg Hudson via RT]

As noted in issue #7062, the PKINIT client code never encodes a TrustedCas 
value as part of a draft9 PA-PK-AS-REQ.  So making changes to the encoder 
should not affect interop.

The PKINIT server code does potentially decode a TrustedCas value when 
decoding a draft9 PA-PK-AS-REQ (if Win2k clients ever send them).  
However, it does nothing with this field during processing.