Have bitcoind recommend a secure RPC password. Increase invalid password delay.

Help users avoid insecure configurations a bit by recommending a
secure RPC password and increasing the incorrect password delay.

This may open up a RPC DOS for users with exposed RPC ports and
short passwords. Since users shouldn't have exposed RPC ports OR
short passwords, the DOS risk is preferable to the compromise
risk.

Also logs the client IP address for incorrect attempts.