Add deprecation notices to publickey(5) tools
Reviewed by: imp, manu
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40144
(cherry picked from commit d5ef73a71f0e2fbc4dc9b907a6e59a15599c50b3)
(cherry picked from commit 84266fb897e6e75569a81418f146f134af04046b)
Approved by: re (gjb)