Copyright and license for the arp.7 manual page

Copyright for arp.7:

This man page is Copyright (C) 1999 Matthew Wilcox <willy@bofh.ai>.

License for arp.7:

Permission is granted to distribute possibly modified copies
of this page provided the header is included verbatim,
and in case of nontrivial modification author and date
of the modification is added to the header.