In this commit, we modify the SignMessage+VerifyMessage slightly in
order to bind the message we're signing to our specific context. We do
this by pre-pending the ascii text "Lightning Signed Message:" to the
msg before we run it through dsha256. Adding this prefix binds the
message to our context in order to avoid accidentally signing a sighash
or the like.
This commit allows users to sign messages with their node's private key
with the SignMessage interface. The signatures are zbase32 encoded for
human readability/paste-ability. Others users can verify that a message
was signed by another node in their channel database with the
VerifyMessage interface.
