To spread efficiently, worms and viruses writers have
to keep their size as small as possible.
So usually scanning large messages (over 2 MB) makes no
sense - we haven't seen infected messages larger than that.
This patch adds support for a new configuration option
to amavis-ng (version 0.1.6.4): sizethreshold,
defaulting to 4M.
Messages larger than that won't be scanned as there's
no need for that and it would only unnecessarily load
the mail server's I/O subsystem.