freshclam.conf.in: make ScriptedUpdates a variable

freshclam.conf.in: make ScriptedUpdates a variable

The 'ScriptedUpdate' setting in freshclam.conf (5) defines whether signature
updates should be fetched incrementally or whether the whole database should
be downloaded.

Since the upgrade of clamav to 0.102.1 led to some problems when downloading
the complete file [0], and some users who switched to incremental updates
later had problems with that, it seems the most comfortable and futureproof
solution is to have a simple way for users to selectively switch to
whichever mechanism works. AFAIR signature downloads of clamav also had
similar problems in previous versions.

Additionally the rendered boolean value was changed from 'yes' to 'true',
to be a bit more consistent with the other booleans in our template.

[0] https://blog.clamav.net/2020/02/clamav-01022-security-patch-released.html

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