Mark `pipex_enable' as atomic. We never check `pipex_enable' within

Mark `pipex_enable' as atomic. We never check `pipex_enable' within
(*if_qstart)() and we don't worry it's not serialized with the rest of
output path. Also we will process already enqueued pipex(4) packets
regardless on `pipex_enable' state.

Use the local copy of `pipex_enable' within pppx_if_output(), otherwise we
loose consistency.

pointed and ok by bluhm@