Reduce output range of ecc_mod_sub.

Reduce output range of ecc_mod_sub.

* ecc-mod-arith.c (ecc_mod_sub): Ensure that if inputs are in the
range 0 <= a, b < 2m, then output is in the same range.
* eccdata.c (output_curve): New outputs ecc_Bm2p and ecc_Bm2q.
* ecc-internal.h (struct ecc_modulo): New member Bm2m (B^size -
2m), needed by ecc_mod_sub. Update all curves.
* testsuite/ecc-mod-arith-test.c: New tests for ecc_mod_add and
ecc_mod_sub.