common/onion is going to need to use this for the case where it finds a blinding
seed inside the TLV. But how it does ecdh is daemon-specific.
We already had this problem for devtools/gossipwith, which supplied a
special hsm_do_ecdh(). This just makes it more general.
So we create a generic ecdh() interface, with a specific implementation
which subdaemons and lightningd can use.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
