pf: Friendly error message for status if pf.ko is not loaded

Check if pf.ko is loaded (i.e. /dev/pf exists) before trying to use it. This
means that '/etc/rc.d/pf status' will no longer return 'pfctl: /dev/pf: No such
file or directory' but 'pf.ko is not loaded'.

PR: 205671
Submitted by: Johannes Jost Meixner <xmj@FreeBSD.org>