Update squid package to 2.6.20 (2.6.STABLE20).
It would be last 2.6 stable release.

Changes to squid-2.6.STABLE20 (25 Apr 2008)

- Bug #2263: Custom log formats fail to log file sizes >2GB properly
  on 32-bit platforms
- Fix stripping NT domain in squid_ldap_group
- Bug #2278: Cache-Control: max-stale=0 forwarded wrongly as max-stale
(without delta)
- Bug #2283: Fails to parse chunked encoding using chunk extensions
- Bug #420: Deal properly with empty list HTTP header members
- Windows Server 2008 support
- Bug #1886: tcp_outgoing_address acl doesn't work with indirect
  source address (follow-x-forwarded-for)
- Bug #2296: Stuck in 100% CPU when fetching an corrupt peer digest
- Add support for the resolv.conf domain directive, and also
  automatically derived default domain
- minimum_icp_query_timeout directive
- Bug #2329: Range header ignored on HIT

(taca)