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 f68ee0e)
(cherry picked from commit 2832af7)

usr.bin/shar/shar.1

@@ -27,12 +27,29 @@
 .\"
 .\"     @(#)shar.1	8.1 (Berkeley) 6/6/93
 .\"
-.Dd January 31, 2019
+.Dd January 1, 2025
 .Dt SHAR 1
 .Os
 .Sh NAME
 .Nm shar
 .Nd create a shell archive of files
+.Sh DEPRECATION NOTICE
+.Nm
+is obsolete and may not be present in
+.Fx 15
+and later.
+Because shell archives are simultaneously data and code and are typically
+interpreted by
+.Xr sh 1 ,
+they can easily be trojan-horsed and pose a significant security risk to users.
+The
+.Xr tar 1
+utility can still produce shar encodings of files if needed.
+The
+.Pa sysutils/freebsd-shar
+port has been created to maintain this version of
+.Nm
+past its deprecation in base.
 .Sh SYNOPSIS
 .Nm
 .Ar