Clarify variable names in cc_keyring.c

Clarify variable names in cc_keyring.c

Consistently use "cache_name" and "cache_id" to talk about the name
and ID of the keyring containing the cache.  In krb5_krcc_resolve, use
"residual" for the residual string as we are no longer using it for
the cache keyring name, and use "anchor_id" for the keyring identified
by the prefix to make it clear that it is not the cache keyring.

Adapted from a patch by simo@redhat.com.