stop appenders losing or leaking what they write

[FreeAndNil]

Fixed

• An oversize event was dropped by the UDP appenders, audit da18b6f-f015
  • the socket rejects a payload over 65507 bytes, measured here: 65507 is sent, 65508 fails
  • new MaxDatagramSize, default 65507, configurable from 512
  • the event is cut on a character boundary, marked ...[truncated] and reported
  • not raisable: IPv6 would carry 20 bytes more, not worth a second limit
• The configurator echoed secret values, audit da18b6f-f042
  • a parameter named like a secret is now logged as *****
  • a connection string keeps only the keywords that name the server
  • covers internal debugging and the four setter failures, which are written without it
  • AdoNetAppender's allowlist moved to log4net.Util.Redact and is now shared
• The pickup mail carried an SMTP DATA terminator, audit da18b6f-f037
  • the file is read to its end, so the lone dot was delivered as a literal line
  • it is gone; body lines stay verbatim and are not dot stuffed
  • an agent that expects DATA framing is documented as not a supported consumer