Default to disable-wallet mode if the wallet doesn't exist

Default to disable-wallet mode if the wallet doesn't exist, and require the user to tell the daemon to create it.
I think this makes sense eventually, and is a further step toward removing the expectation that Bitcoin Core is a wallet.

Add a command line option `-createwallet` to create the wallet. Passing this option when the wallet already exists does nothing.

N.B.: The GUI still defaults to creating the wallet by default. There would first need to be a user-friendly way to create the wallet to change this behavior, for example through a menu option. This is going to be the
next step if the idea is accepted.