man dummynet: point to dnctl instead of ipfw

Dummynet configuration is ideally done through dnctl now. While ipfw
still works dnctl is preferred now that dummynet can also be used with
pf.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31902