pf: store L4 headers in pf_pdesc

Rather than pointers to the headers store full copies. This brings us
slightly closer to what OpenBSD does, and also makes more sense than
storing pointers to stack variable copies of the headers.

Reviewed by: donner, scottl
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30719