Add a maximum class number value to ECPP.

Paul Underwood has reported a case where the previous code would use
D = -15963964631 = -17 * 29 * 31 * 41 * 73 * 349 with class number
249984 = 2^7 * 3^2 * 7 * 31, which is smooth, but too big.

* lib/ecpp.c (compute_discriminants, compute_sorted_discriminants,
  expected_no_curves, find_ecpp_discriminant): Add parameter hmax and
  adapt function calls.
  (ecpp1): Set hmax to 100000 and adapt function call. Output the class
  number in verbose mode.