config: freshclam: default to incremental downloads

config: freshclam: default to incremental downloads

clamav recently started yielding 429 (too many requests) response
codes on even comparatively low attempts to download the complete
signature files (cvd)(see [0]), instead of the incremental changes
(cdiff) (see [1] for some background)

changing the default to scriptedupdates (a.k.a. cdiff download) seems
sensible for most situations.

[0] https://docs.clamav.net/faq/faq-freshclam.html
[1] https://blog.clamav.net/2021/03/clamav-cvds-cdiffs-and-magic-behind.html

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