Number of main Squid processes or "workers" to fork and maintain.
0: "no daemon" mode, like running "squid -N ..."
1: "no SMP" mode, start one main Squid process daemon (default)
N: start N main Squid process daemons (i.e., SMP mode)
In SMP mode, each worker does nearly all what a single Squid daemon
does (e.g., listen on http_port and forward HTTP requests).
Changing the number of workers requires a restart: Squid warns about but
otherwise ignores attempts to change this setting via reconfiguration.