Log Message:
Update to mimedefang 2.56

Changes 2.56:
An off-by-one error in the multiplexor that could restart slaves
unnecessarily was fixed. Compilation errors on some systems were fixed.
A handful of other minor bugs were fixed.

Changes 2.55:
A new option allows you to reserve some slaves for connections from
localhost; this helps clientmqueue runs to succeed on busy servers.
Modern Vexira anti-virus scanners are supported; versions older than
Spring 2005 are no longer supported. A new "filter_helo" callback lets
you take action in response to HELO/EHLO. A new "action_insert_header"
function lets you prepend headers (rather than just appending them).
A new function lets you reject mail from hosts with bogus MX records;
for example, MX records that resolve to private IP networks or the
loopback address.