Replace proxy_monitor with sqpmon for squid3

See forum thread http://forum.pfsense.org/index.php/topic,57374.0.html
Help a user who has problems downloading files with "proxy" in the name.
Also, proxy_monitor.sh was in /usr/local/etc/rc.d - this is supposed to be a dir just for service start/stop scripts that take "start" or "stop" as a parameter. When rc.stop_packages calls "proxy_monitor.sh stop", proxy_monitor does not actually get stopped. At system shutdown this leaves proxy_monitor running, and it may wake up before the system actually shuts down, do its check for squid, discover squid is not running, and start squid again! It would be best if this scenario could not happen.
This change puts sqpmon.sh in /usr/local/pkg from where it can run, like a "service". squid.inc has install code to write_rcfile sqp_monitor.sh to /usr/local/etc/rc.d - sqp_monitor.sh is a standard-format start/stop script that starts/stops sqpmon.sh - this makes it all start/stop in the standard manner. 
"sqpmon" = "SQuid Proxy MONitor" - this makes a unique-looking name that won't match "squid", "squidGuard", "squid3", "squid-reverse"...
Once this is confirmed to be OK, I will also make the same changes to squid(2) so that they are consistent, avoiding future confusion.