squid: security bump to version 3.5.17

Fixes:
SQUID-2016:5 (CVE-2016-4051) - Buffer overflow in cachemgr.cgi
SQUID-2016:6 (CVE-2016-4052) - Multiple issues in ESI processing.
CVE-2016-4053 & CVE-2016-4054 which are part of SQUID-2016:6.

Switch to xz-compressed tarball as well.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>