pf: fix netpfil.common.dummynet:pf_nat test

This test failed if ipfw was loaded (as well as pf). pf used the same
tag as dummynet to indicate a packet had already gone through dummynet.
However, ipfw removes this tag, so pf didn't realise the packet had
already gone through dummynet.

Introduce a separate flag, in the existing pf mtag rather than re-using
the ipfw tag. There were no free flag bits, but PF_TAG_FRAGCACHE is no
longer used so its bit can be re-purposed.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33087