pf: factor out pf_setup_pdesc()

factor our the code to set up pf_pdesc, a central structure in pf carrying
information about the packet we're currently dealing with, into its own
function. ok ryan dlg and additional testing sthen

Obtained from: OpenBSD, henning <henning@openbsd.org>, c4202972a3
Obtained from: OpenBSD, claudio <claudio@openbsd.org>, 78d25123ea
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46586