Proper ISMINE_WITNESS behaviour

[mjmacleod]

When we have only the witness key for a witness address and not the spending key, the codebase should behave differently.

The first part of this is already handled the various 'ismine' functions return ISMINE_WITNESS; however presently the rest of the codebase doesn't know how to deal with this and treats it as ISMINE_SPEND (aside from various UI components that treat things differntly at an account level, and a few RPC commands that know better.)

The rest of the codebase should be fixed to handle this correctly.

See: 3c1381b