net/pbnc: Fix license information

License of this software is declared as 'BSD' in pbnc.readme.
But it is not clear which versions of the BSD license (2, 3 or 4 clause)
it is. No further file in the source archive contains license information.

Committed with minor changes like done in r462831.

PR: 222546
Submitted by: Yasuhiro KIMURA <yasu@utahime.org> (original version)
Approved by: mentors (implicit)
Differential Revision: https://reviews.freebsd.org/D17662