pfctl: add option -S (no domain resolution)

manpage wording and reminder about usage() jmc@
ok florian@ henning@

Reviewed by: ziaee (manpages)
Obtained from: OpenBSD, benno <benno@openbsd.org>, 7c8726d43b
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D50724