Introduce DISABLE_FRESHCLAM

When set, freshclam won't be started. This feature makes sense only in testing enviroment with limited resources: the official databases are big and clamd loads them in memory at startup. However clamd won't start without valid databases so make sure there's at least one (urlhaus).

conf/chaperone.d/freshclam.conf

 freshclam.service: {
     command: "/usr/bin/freshclam -d --foreground=true",
+    enabled: "$(DISABLE_FRESHCLAM:+false)",
     exit_kills: true,
 }