gss krb5 mech enhanced error messages

gss krb5 mech enhanced error messages

Save detailed error messages (usually from the krb5 library) in
per-thread storage, mapping each error code to the most recently
produced message for it.  Return the message from display_status.

Currently not implemented for a few cases where the krb5 mechanism
returns a minor status code of 0, or another value different from the
libkrb5 error code.

Other functions are available to store a generic string or formatted
message, but aren't used much at present.

Tested with these errors in context establishment:
 * missing ccache (libkrb5 shows pathname if FILE: type)
 * missing keytab (libkrb5 shows pathname if FILE: type)
 * server principal unknown (libkrb5 shows server principal)

ticket: new

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@19672 dc483132-0cff-0310-8789-dd5450dbe970