Allow S4U2Proxy delegated credentials to be saved

Allow S4U2Proxy delegated credentials to be saved

The initial implementation of client-side S4U2Proxy support did not
allow delegated proxy credentials to be stored (gss_store_cred would
error out, and gss_krb5_copy_ccache would generate a non-working
cache).  To make this work, we save the impersonator name in a cache
config variable and in a cred structure field (replacing the
proxy_cred flag), and make the default principal of the proxy cache
the subject principal as the caller would expect for a regular
delegated cred.

ticket: 7046

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25529 dc483132-0cff-0310-8789-dd5450dbe970