ftp/tnftp: Add license

[1]
tnftp contains a mix of BSD2CLAUSE (mostly the NetBSD
derived code) and BSD3CLAUSE (inherited from the orignal UCB BSD code).

tnftp was previously BSD4CLAUSE but that was fixed in the upstream code
a while ago so I changed the COPYING to reflect that. It's no more
restrictive than BSD3CLAUSE now.

Suggested by: [1] Luke Mewburn <lukem@NetBSD.org>
Suggested by: rodrigo, salvadore