Bug 2066: squid does not do chdir() after chroot()

Bug 2066: squid does not do chdir() after chroot()

The earlier workaround applied only fixed 1 of the 3 places performing
chroot().
 This makes chroot and chdir integral parts of setting up Squids running
directory and alters teh chroot() calls to be mainSetCwnd() calls. Which
fixes several potential problems with core dumps from squid -z or -k
executions ending up in unexpected locations, regardless of whether
chroot() and coredump_dir are configured.

 Detected by Coverity Scan. Issue 740335.