squid: Update to 5.1

squid: Update to 5.1

For details see:
http://www.squid-cache.org/Versions/v5/changesets/

There is still no official announcement.

Nevertheless, since 31 Jul 2021, 'squid 5.1' has become "stable"
and is listed under "Current versions suitable for production use".

The only problem I found during testing deals with 'privoxy'.

Since 'privoxy' - as parent cache_peer - sometimes replies with a '403',
'squid 5.1' handles this cache_peer connection as 'dead' which is then
logged in 'cache_log'. See discussion on list.

Actually this is something that got fixed from 'squid 4.16' to '5.1' - its
no bug - its a feature. Everything else works as expected,'squid' and
'privoxy' developers were informed.

Signed-off-by: Matthias Fischer <matthias.fischer@ipfire.org>
Reviewed-by: Peter Müller <peter.mueller@ipfire.org>