Make checkpointing dependent on CM_ECPP_TMPDIR.

* lib/cm-impl.h (mpzx_oneroot_split_mod, cm_class_get_j_mod_p,
  cm_curve_and_point_stat, cm_ecpp_one_step2,
  cm_mpi_submit_ecpp_one_step2): Add tmpdir parameter.
* lib/curve.c (cm_curve_and_point_stat, cm_curve_and_point): Add tmpdir
  parameter and adapt function calls.
* lib/ecpp.c (ecpp2, cm_ecpp_one_step2, cm_ecpp): Add tmpdir parameter
  and adapt function calls.
* lib/jmodp.c (get_root_mod_p, get_tower_root_mod_p,
  get_quadratic_tower_root_mod_p, cm_class_get_j_mod_p): Add tmpdir
  parameter and adapt function calls.
* lib/mpi.c (cm_mpi_submit_ecpp_one_step2): Add tmpdir parameter and send
  it to workers.
  (mpi_worker): Receive tmpdir parameter.
* lib/pari.c (mpzx_oneroot_split_mod): Add and treat tmpdir parameter.