Copyright for biff source package in Xenial

This version of biff was obtained from:
ftp.uk.linux.org:/pub/linux/Networking/netkit.

It has been packaged by Martin Mitchell  who added
the debian/* files and added miscellaneous patches for compilation.
Dave Holland  made other changes.

The biff package is distributed under the BSD copyright which can be
found in /usr/share/common-licenses/BSD on a Debian GNU/Linux system.