Rework the logic in ECDSA sign_sig()

Rework the logic in ECDSA sign_sig()

If the caller supplied both kinv and r, we don't loop but rather throw
an undocumented error code that no one uses, which is intended to tell
the caller to run ECDSA_sign_setup() and try again.

Use a boolean that indicates this situation so that the logic becomes
a bit more transparent.

ok jsing