Allow PID file to be disabled at compile-time

[chrisnovakovic]

Stubby unconditionally attempts to write a PID file under the run state directory when it starts, but this directory is not always writeable by non-root or non-system account users (e.g. on Slackware). To aid with running Stubby in environments where the PID file would otherwise be unwriteable or unnecessary, allow it to be disabled at compile-time with the ENABLE_PID_FILE option (which remains on by default).