Add code for testing ECPP certificates.

The implementation can check partial certificates and also apply the
stricter criterion of the CM certificates compared to the PARI/GP
(and apparently Primo) certificates that the point order is prime.

* lib/pari.c (cert_get_V, primecertentryisvalid, primecertpartialisvalid):
  New functions.
  (cm_pari_ecpp_check): Use primecertpartialisvalid.