- Fix SASL "Authentication failed" message to include EMSS code

- Tests:
  - Test application of environment variables in mail rules

- Add hooks for plugins to load or (temporarily) disable the operation
  of other plugins?

Plugin: starttls

  - Run TLS handling in a subprocess for isolation?

Plugin: clamav

  - Add tests (not sure how to emulate the daemon properly)

Plugin: lua

  - Fix hooks that can modify strings to actually allow modification

Plugin: patterns

  - Add better MIME attachment discrimination to patterns, instead of
    just keying on a blank line.

Plugin: mailrules

  - Add variable substitution to rule variable setting (ie x=${y} etc)

  - Allow selecting based on authentication state.

Protocol: SMTP

  - Build a shared routine for breaking out the domain name.

  - Add RFC 2034/3463 enhanced status codes to all responses.

  - Add some information about authentication into the headers (?)

  - Handle $LOCALIPHOST (all back-ends): rewrite envelope recipient
    addresses of the form box@[a.b.c.d], where a.b.c.d is a local IP
    address, to box@$LOCALIPHOST. (low priority)

- pop3front:

  - Properly re-parse flags in cmd_quit

  - Split the generic protocol handling bits from pop3front-maildir (low
    priority -- who wants anything but maildir?)