[krbdev.mit.edu #6150] KerberosAgent Identity input should autocomplete to valid principal

[Justin Anderson via RT]

Enter Identity's "Identity:" NSComboBox and Select Identity's new identity sheet's "Identity:" 
NSTextField should suggest validated versions of their contents when there isn't anything 
matching in the favorites. 

e.g. "user" would suggest "user at ATHENA.MIT.EDU" as long as nothing in the favorites starts 
with "user"