Switch sign_sig() and sign_setup() to using BN_CTX

Switch sign_sig() and sign_setup() to using BN_CTX

Both these functions use a BN_CTX internally to deal with the EC API
that usually requires one. However, they don't actually make use of it.
Get the BIGNUMs from the BN_CTX instead, which simplifies the cleanup.
Also defer allocation of the ECDSA_SIG to the very end. Instead of using
its internal r and s, use two local r and s variables and transfer those
to the ECDSA_SIG on success.

ok beck jsing