Defer KEYRING key creation until initialize

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.