[Mimedefang] Filter_initialize/cleanup persistent handle in slave caveat.

[Ray Ferguson]

I added this to the wiki faq.  It seemed tricky enough to warrent passing 
along.

http://www.mimedefang.org/kwiki/index.cgi?FilterCleanup

If you read the mimedefang-filter man page, you will see that filter_cleanup 
is called on exit, but that is just the normal case.

A mimedefang-multiplexor slave-busy-time-out results in a term signal without 
filter_cleanup.

To further complicate matters, the embedded perl note in the man page 
regarding filter_initialize and the scope of handles also applies to signal 
handlers. That is to say you need to add:

  $SIG{"TERM"} = \&filter_cleanup ; 

To filter_initialize() where you open the handles if you need to make sure 
they are consistently closed on exit in filter_cleanup.

-- 
Ray Ferguson