Manual rebase of Shannon's PR-294 rewrite.

This got a little messy as some changes now needed to be applied in
two places, and other wording has been completely removed.  Another
pass on top will be required.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: Shannon Appelcline

.aspell.en.pws

@@ -301,3 +301,5 @@ localhtlcsig
 basepoints
 Bitcoins
 deobfuscate
+offerer
+offerer's