RFC: simplifications and suggestions on open/accept limits.

While it is possible to negotiate not to accept HLTCs or channel balances below 1000 msat, there is no way to negotiate to only accept multiples of 1000 msat (whole satoshis). The user suggests adding either transaction_min_msat_multiple (set to 1000 for only accepting whole satoshis) or accept_subsatoshi (1/0) to allow users to opt out of sub-satoshi payments. The email also discusses suggestions for other negotiation values. For example, funding_satoshis should not be too small to avoid being consumed by fees, and Eclair's suggestion of a minimum of 1000 msat for htlc_minimum_msat was defended as important protection against trimmed HTLCs. The email suggested dropping max_htlc_value_in_flight_msat, requiring channel_reserve_satoshis be 1%, and setting minimum_depth to be chain-dependent. There were also suggestions for dust_limit_satoshis and max_accepted_htlcs.