IFP: Initialize negative cache timeout
    
    In order to avoid hitting the back end with repetitive requests, the
    InfoPipe responder needs a negative cache, too. This patch follows the
    convention set by other responders, where the negative cache timeouts are
    read from the [nss] section. This is not ideal, however, and ticket #2318
    tracks moving the configuration to the [ifp] section primarily.
    
    The timeout is also a separate parameter in the NSS context. We should
    consider moving it to the negcache context instead (#2317).
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Stef Walter <stefw@redhat.com>
    (cherry picked from commit 6cbb9f0d7c6be2cd3553dcb548984bb98926d5cb)