Implemented private repo feature in freshclam

[arunbhagyanath]

Using freshclam's Private mirror feature

This option allows you to easily point freshclam to private mirrors.
If PrivateMirror is set, freshclam does not attempt to use DNS
to determine whether its databases are out-of-date, instead it will
use the If-Modified-Since request or directly check the headers of the
remote database files. For each database, freshclam first attempts
to download the CLD file. If that fails, it tries to download the
CVD file. This option overrides DatabaseMirror, DNSDatabaseInfo
and ScriptedUpdates. It can be used multiple times to provide
fall-back mirrors.