Changes to cred store design

[Nico Williams]

That leaves the "cred_store" in... just the argument names?  Might I
suggest that perhaps "cred store" was the wrong analogy[*] maybe we
should call this "raw_cred"?  Note that such a change would not impact
the implementation nor the ABI, just the docs.

[*] It's fine for referring to ccaches and acceptor keytabs, but not
fine for referring to password files and initiator keytabs.  Cred
store was my original design choice.