NSS: add support for SSS_NSS_EX_FLAG_INVALIDATE_CACHE
The patch adds support for the SSS_NSS_EX_FLAG_INVALIDATE_CACHE flag and
makes the existing code more flexible and handle additional flags.
If SSS_NSS_EX_FLAG_INVALIDATE_CACHE is set the requested object is only
looked up in the cache and if it was found on-disk and memory cache
entries will be invalidated.
Related to https://pagure.io/SSSD/sssd/issue/2478
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>