miner: provide coinbase when starting the miner

This error can no longer be returned from eth.New.

This avoids having to query the coinbase when creating the miner, which
in turn eliminates the dreaded startup error when no accounts are set
up. Later, this will also allow us to simply restart the miner when the
user picks a different coinbase.

This causes a lot of changes in other packages. These are included in
this commit because they're impossible to separate.

This is mostly for automated tests. The tests can use the following
commands to start the node:

    ethereum --unencrypted-keys account new
    ...
    ethereum --unencrypted-keys

Conflicts:
	rpc/api.go
	rpc/args.go

Accounts integration