More on bug: when signing txs with SW and non-SW inputs, the
existing signature_form for non-SW inputs didnt pay attention
to whether the tx serialization was SW or non-SW style, so if
a SW-style tx serialization was passed in, it was kept in SW
form after passing through signature_form, resulting in an attempt
to create a signature based on SW style sighashing. The fix here
just ensures that signature_form always enforces the tx format
to be non-SW style, so allowing callers to sign() to get a valid
signature whatever they pass in.
AdamISZ