Defer KEYRING key creation until initialize

If we resolve a KEYRING cache and the key does not exist, wait until
initialize time to create it, to avoid wasting precious kernel memory
on a cache which might not ever be created.  Properly error out if
store_cred or start_seq_get is called on an uninitialized cache, as we
would for a FILE cache.

Adapted from a patch by simo@redhat.com.
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.