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)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D48130