The function ipip_output() was registered as .xf_output() xform

The function ipip_output() was registered as .xf_output() xform
function.  But was is never called via this pointer.  It would have
immediatley crashed as mp is always NULL when called via .xf_output().
Do not set .xf_output to ipip_output.  This allows to pass only the
parameters which are actually needed and the control flow is clearer.
OK mpi@