Audit all NSE scripts to document protocols/scripts with insecure auth methods

Nmap has more than 600 NSE Scripts and a decent number are able to accept authentication credentials. Many protocols (particularly legacy ones like telnet, ftp, or (non TLS) http) transmit them in cleartext or otherwise aren't up to modern security standards. Some users may not be aware of these risks for some protocols. There are also cases where a protocol may support multiple authentication methods, and Nmap may not support the most secure ones. As a first step, we should identify all the NSE scripts which accept usernames and password arguments, check for this sort of security issue in the protocol itself and/or how Nmap implements it. And then at least document the risk in the NSEDoc for the script so that script users understand the risks of providing auth in these cases. In some cases, this might be a good chance to upgrade the script to support newer authentication methods for the protocol that may be more secure.