NEGCACHE: initialize UPN negative cache as well
UPNs are handled separately in the negative cache. To properly filter
user names even in the case of the fallback to a UPN lookup the negative
cahe for UPNs has to be initialized with the names from the filter_user
option as well.
If the name from the option is a short name it will be added to the
negative UPN cache for each domain with the respective domain name. If
the name from the option is fully-qualified it will be added as is to
the negative UPN cache for each domain.
Related to https://pagure.io/SSSD/sssd/issue/3978
Reviewed-by: Jakub Hrozek <email@example.com>