[pf4freebsd] Some more changes to the port

[Max Laier]

Hello,

Pyun identified some minor problems with the port and Chris pointed out,
that some of the includefiles are needed in order to build squid
with --enable-pf-transparent

Changes:
    1) Include files are installed to ${PREFIX}/include/pf (For building
squid et al, you'll have to set your CFLAGS accordingly)
        Such things should go to a short document in PORTSDOC. Is somebody
willing to write such???
    2) The boot script explain why it was not able to determine PREFIX (use
full path)
    3) The modified manpage for pftcpdump.1 is installed.
    4) Fixed a pkg-plist error I introduced when incooperating Chris' last
comment about pf.conf.default

I guess the port is now pretty ready for submit ... please tell me ASAP if
you find anything that troubles you.
Thanks a lot to Chris and Pyun.
    Max