We need to control the *inputs* to the anchor tx, to make sure they
pay to witness scripts (thus the anchor is immalleable). The easiest
way to do this is to hand out P2SH addresses for the user, and have
them pay into those. Then they hand us that tx and we use it to
create the anchor.
This is not a long-term solution!
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
