www/squid: implement proper backtrace support

Now Squid will print full backtrace for problematic cases when it is
about to die on some signal and should emit stacktrace. You'll
obviously need to compile it with STACKTRACES option.

Tested at: KIAE Grid production Squid cluster
MFC after: 1 week
MFH: 2014Q4
Sponsored by: Kurchatov Institute of Atomic Energy