Log Message:
* Fixed a bug in embedded Perl: We have to call
  PERL_SET_CONTEXT after forking or Perl gets confused.
  In particular, setting signal-handling dispositions using
  $SIG{FOO} = sub { ... } breaks.
* Clarify wording of mimedefang-filter man page.
* Remove obsolete code that used to attempt to generate working
  directory names.  Deactivate the no-longer-needed "-M" mimedefang
  option.
* Add new "-y" option to mimedefang-multiplexor.  This limits
  the number of concurrent "recipok" commands on a per-domain basis.
* Remove Anomy::HTMLCleaner support.
* use MIME::Parser::Filer's ignore_filename() call instead of
  subclassing to override evil_filename().  Same effect, less code.
* refactor resend_message_one_recipient() to use
  resend_message_specifying_mode() instead of reimplementing it.
* header_timezone() now generates a strictly RFC2822-compliant timezone
  string without needing POSIX::strftime()
* Ensure that decode_mimewords() is called in scalar context.