shar: add a deprecation notice

The shar(1) program is simple, but the fundamental idea of a sh archive
is risky at best and one that we probably shouldn't be promoting as
prominently as a program in $PATH and a manpage. Let's deprecate and
remove it, since the same functionality can easily be found in
tar(1) instead.

Reviewed by: emaste, philip
Reviewed by: allanjude, brooks, delphij, des, imp, rpokala (previous)

(cherry picked from commit f68ee0e7a1e8732f725cad4ac708ec49093782d4)
(cherry picked from commit 2832af7b4ea256b18ef4dbf2ff97a50765f0609a)