pf: support dummynet on L2 rules

Allow packets to be tagged with dummynet information. Note that we do
not apply dummynet shaping on the L2 traffic, but instead mark it for
dummynet processing in the L3 code. This is the same approach as we take
for ALTQ.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32222