This provides a speed-up when several rounds are needed in the main loop
for determining a CM discriminant, and in the MPI version it lets the
clients idle more.

* lib/cm-impl.h (cm_mpi_broadcast_N): Add parameters e, r and z to
  prototype.
* lib/ecpp.c (compute_qroot): Add e, r and z as parameters to the non-MPI
  version and do not compute them.
  (find_ecpp_discriminant): Compute e, r and z and adapt function calls.
* lib/mpi.c (cm_mpi_broadcast_N): Broadcast e, r and z.
  (mpi_worker): Receive e, r and z.