We alternated between using a sha256 and using a privkey, but there are
numerous places where we have a random 32 bytes which are neither.
This fixes many of them (plus, struct privkey is now defined in terms of
struct secret).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This is a bit tricky: for our signing code, we don't want scriptsigs,
but to calculate the txid, we need them. For most transactions in lightning,
they're pure segwit so it doesn't matter, but funding transactions can
have P2SH-wrapped P2WPKH inputs.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
