Allow path-bound builtins on restricted shells

If a system administrator prefixes /opt/ast/bin to the path and
then invokes the shell in restricted mode, they clearly intend for
the user to run those AST utilities.

Similarly, if a system administrator sets a PATH for a restricted
shell that includes libraries listed in the .paths file, they must
have intended for the user to use those loadable built-ins, as they
will be associated with the pathnames of their respective
libraries. Since the user cannot change PATH or use the builtin
command, they still cannot load just any built-in they choose.

src/cmd/ksh93/sh/path.c:
- Remove SH_RESTRICTED check when handling path-bound builtins
  or dynamic libaries containining builtins in $PATH.

src/cmd/ksh93/tests/builtins.sh:
- Add test verifying a restricted user can use /opt/ast/bin/cat
  via a PATH search.

Progresses: https://github.com/ksh93/ksh/issues/138

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-04-08:
+
+- Path-bound builtins will now be used by restricted shells if /opt/ast/bin
+  is in the $PATH upon invoking the shell or before setting it to restricted.
+
 2021-04-07:
 
 - The $LC_TIME variable is now recognized by ksh and if set to an invalid