New port: net/libpfctl: library for interaction with pf(4)

The libpfctl port builds a shared library version of the base system
internal libpfctl library.
While the base system libpfctl API/ABI is not guaranteed to be stable
the ioctl interface is, so any version of libpfctl can be safely used.

Note that currently only FreeBSD main (i.e. what will become 14)
supports libpfctl. The relevant changes will be MFC'd back to stable/13
and stable/12 in due course. At that point libpfctl will also be useful
there.

Approved by: garga
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29836