Remove support for PF (Packet Filter)

Remove support for PF (Packet Filter)

OpenVPN shipped a small packet filtering tool called PF. It has never
been straightforward as it required a plugin to work. On top of that,
keeping PF support, makes the code more complicated and increases the
maintenance cost of OpenVPN.

PF itself is not actually maintained at all and there is little
motivation in keeping it alive.

Some years ago an IPv6 extension for PF was proposed, but it was never
picked up for the reasons above.

External (and more appropriate) tools can still be used to implement
packet filtering on the OpenVPN interface.

Drop PF support for good.

Note that IDs used for external communication (i.e. to the plugin
or management interface) have been commented out, but not removed, as
they should not be used in the future.

v2:
* changed // to /* */
* changed "NOT IMPLEMENTED" to "REMOVED FEATURE"
* removed extra empty lines after removing ifdef blocks
* clarified on IRC that tls_final has to be removed and therefore that
  hunk is correct
* removed mi_prefix() function as it is now unused

Cc: Arne Schwabe <arne@rfc2549.org>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20210827190014.12640-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22780.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>