IFP: Change ifp_list_ctx_remaining_capacity() return type
    
    Now ifp_list_ctx_remaining_capacity() returns an errno_t and receives
    the count as an output parameter. It allows better handling and error
    reporting in case something goes wrong internally in this function.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3306
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>