Leave room for CPFP in 0FC Max HTLCs

[TheBlueMatt]

Iirc the spec limit can lead to a commitment tx that's exactly 10kvb and thus can't be CPFP'd. I guess this is actually an issue with general anchor channels too but we need to be more conservative for 0FC. Also probably need to update the spec?

[tankyleo]

• A 0FC commit tx with 114 * 2 HTLCs has 9997vB, smaller than max 10'000vB for any TRUC.
• We get a separate allocation of 1000vB for the anchor tx (due to unconfirmed parent).