pmg-smtp-filter: die if processing took longer than the timeout

pmg-smtp-filter: die if processing took longer than the timeout

In case a mail took longer to get processed than the configured
timeout - 1 second - `die` before running any action.

The `die` results in a temporary failure to be reported to the sending
server by PMG::SMTP.pm ("451 4.4.0 detected undelivered mail").

The reason for the 1s extra slack is to have some time to actually
run the action - and also justified that in both cases (postfix
detecting the timeout, and pmg-smtp-filter `die`ing the sender gets
a temporary failure reported back).

Tested with a small filter_timeout setting (30), and a larger sleep in
added in analyze_virus.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by:  Dominik Csapak <d.csapak@proxmox.com>