pf can use dummynet for packet scheduling, but the dummynet module
currently depends on ipfw. We don't want to break this dependency as
that'd risk breaking user configurations (i.e. where those users rely on
the dependency so that loading dummynet also loads ipfw).

For stable/13 and stable/12 we introduce pfdummynet, which is the
dummynet module with a dependency on pf instead of on ipfw). In later
versions the dummynet module has no dependencies, so can be loaded
separately from a firewall.

Note that pfdummynet.ko and dummynet.ko cannot be loaded simultaneously.

Sponsored by: Rubicon Communications, LLC ("Netgate")