Make krb5_cc_new_unique create DIR: directories

When we use krb5_cc_new_unique to create a new cache in a directory
cache collection, we will fail if the directory doesn't exist yet.

Go ahead and preemptively create it, as we do during krb5_cc_resolve,
before attempting to create a new file under it.

ticket: 7988 (new)
target_version: 1.13
tags: pullup