Clean up and simplify EC_KEY handling, mostly from a BN_CTX perspective.

Clean up and simplify EC_KEY handling, mostly from a BN_CTX perspective.

If we have a BN_CTX available, make use of it rather than calling BN_new().
Always allocate a new priv_key and pub_key, rather than having complex
reuse dances on entry and exit. Add missing BN_CTX_start()/BN_CTX_end()
calls.

ok tb@