Bedrock shouldn't break (unbreakable/unplaceable block types)

[satoshinm]

There should be some types of blocks the client cannot break, such as bedrock (and/or barrier?).

[satoshinm]

This could benefit from client changes. But should the client never be able to break bedrock, yet still place it? (what if they get stuck?) If not, this implies some kind of permissions system.