```
make check-source-bolt CHECK_BOLT_PREFIX="--prefix=BOLT-onion-message" BOLTVERSION=guilt/offers
```
Mainly textual, though I neatened the extra fields check for TLVs with
blinding, and implemented the "no other fields" requirement for
non-final onion message hops.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>