Consider disallowing bare send of value to singleton actors

From @raulk:
Right now it's possible for account actors to perform naked sends (method num = 0) to singleton actors (e.g. system, market, power, etc.), from which the funds will never be rescued (unless a future fork/protocol change unlocks/returns them).

At least in Ethereum there is the minuscule chance that one can crack the private key for account 0x000000...00

But in Filecoin, singleton actors don't even have private keys associated to them. So these funds have nil chance of being released, ever.

Is there a use case behind allowing method_num = 0 messages to singleton actors? If not, I suggest we explicitly reject these messages.

Fun fact: account 0x000000....00 in Eth1 has 3M USD in balance from transfers made, most certainly, by mistake (or poor UX of wallets). Let's remove this footgun from our system.