Move frontier-squid.sh from /etc/init.d to /usr/libexec/squid

To resolve issue #216, this merge request changes the RPM spec file to move the location for the frontier-squid.sh script from /etc/init.d to /usr/libexec/squid.

In addition, commands formerly in the frontier-squid-utils makefile to prepare prototype files have been moved into the RPM spec file.

Note that direct upgrades from frontier-squid-4.13-5.2 or earlier may require a manual sudo /sbin/chkconfig --del frontier-squid operation, but one hopes that users are all on later versions of frontier-squid by now.

This change was tested by creating the RPM, installing it, and checking that systemctl start/stop/reload commands were able to control frontier-squid. Also, the installation was checked to verify that the files from frontier-squid-utils were properly installed in the correct locations.