Split ec_asn1_parameters2group() into digestible pieces

Split ec_asn1_parameters2group() into digestible pieces

This becomes a simple wrapper function that currently does three checks:

1. ensure the fieldID is for a prime field

2. check that the purported prime is of reasonable size, extract and
   set curve coefficients and point conversion form

3. extract and set generator, order, cofactor and seed.

Sanity checks such as the Hasse bound are dealt with in the EC_GROUP API,
so need not be repeated here. They will become redundant once we enforce
that the parameters represent a builtin curve anyway.

ok jsing