Needed enhancements to pop3-brute.nse

[dmiller-nmap]

pop3-brute.nse is a very old script, and it's in need of some upgrades.

• Determine if STLS is required before authenticating and perform it. See https://seclists.org/nmap-dev/2010/q2/156
• Determine if the auth method being attempted is supported, to avoid doing work when it won't succeed.
• Attempt to get a list of supported auth methods and choose one automatically. Sending AUTH with no options sometimes works for this, as well as finding supported extensions with CAPA. Maybe extend pop3-capabilities to do this also?
• Support NTLM authentication (see pop3-ntlm-info and smb-brute)
• Use extended error codes (RFC 2449 and RFC 3206) to identify when an -ERR might actually mean a successful authentication (e.g. IN-USE), a need to slow down (e.g. LOGIN-DELAY, see ftp-brute), or a need to retry (e.g. SYS/TEMP), among others.