ncache: Silence critical error from filter_users when default_domain_suffix is set
    
    When default_domain_suffix is used and filter_users is set (at least
    root is always, by default), SSSD tried to add the negcache entry to the
    default domain. But since the default domain is not known after start
    up, adding the entries fail with a verbose error message.
    
    This patch handles EAGAIN returned from the parsing function while
    setting negcache entries gracefully and also makes the debug message in
    parsing function more precise.
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit 1aa492ce890f362564bfac21f3cfb0a3e38608bd)