scripts: improve CLAMAV_MESSAGE_SIZE_LIMIT usage (#3332)

* add sanity check for Clam size & adjusted MaxScanSize

The second part is of special importance! See
<https://askubuntu.com/a/1448525>, which explains that the maximum scan
size is important as well. We previously just set the maximum file size,
which actually is pretty insecure as we silently not scan mile bigger
than `MaxScanSize`. This is corrected now.

* add SlamAV size configuration to Rspamd

target/rspamd/local.d/antivirus.conf

@@ -10,4 +10,5 @@ ClamAV {
     scan_mime_parts = false;
     symbol = "CLAM_VIRUS";
     log_clean = true;
+    max_size = 25000000;
 }