adapt clamav.conf.in to new upstream version

adapt clamav.conf.in to new upstream version

The new version of clamav has changed a few options, which we ship in our
config template - see [0] for the details.
This patch changes all affected options to the new names and additionally
expands the Description of 'archiveblockencrypted' to reflect that the option
is used for both archives _and_ documents.

[0] https://blog.clamav.net/2018/12/clamav-01010-has-been-released.html

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>