Add a SECURE style flag for ThreadSafeMessageBox, which indicates that the message contains sensitive information

This keeps the message from being output to the debug log by bitcoind. Fixes a possible security risk when starting bitcoind in server mode without the 'rpcpassword' option configured, resulting in the "suggested" password being output to the debug log.

Fixes #5094.
Whoops, yep. The first version had that line inside an `if (fSecure)` block. Fixing...