Tidy up of wallet locking and password RPC commands

[dsiganos]

Tidy up of wallet locking and password RPC commands.
The following should be done:

• password_valid: deprecated in favor of existing wallet_locked (which implies there is a valid password existing already)
• wallet_key_valid: deprecated in favor of existing wallet_locked (which implies there is a valid password existing already)
• password_change: deprecated in favor of new alias wallet_password_set
• password_enter: deprecated in favor of new alias wallet_unlock

[dsiganos]

Example use case:
Set a password for a wallet with wallet_password_set
Lock a wallet (using the password set above) with wallet_lock
Unlock a wallet with wallet_unlock
Check if a wallet is locked with wallet_locked

[zhyatt]

@dsiganos After further review, the password_valid/wallet_key_valid calls will return "valid":"0" if there is no password set on the wallet. That information is not otherwise available so we need to consider whether it should be kept in some form, as wallet_lock currently returns "locked":"1" even when no password is not set (and the wallet thus cannot be locked). So I guess two things:

• Fix wallet_lock to return something other than "locked":"1" when there is no password
• Perhaps keep one of the valid calls and rename it to wallet_password_exists to be more explicit about its use

Thoughts?