Fix the readonly builtin's scope in functions (#51)

* Fix the readonly builtin's scope in functions

This bug was first reported at https://github.com/att/ast/issues/881

'tdata.sh->prefix' is only set to the correct value when
'b_readonly' is called as 'export', which breaks 'readonly' in
functions because the correct scope isn't set. As a result, the
following example will only print a newline:

$ function show_bar { readonly foo=bar; echo $foo; }; show_bar

The fix is to move the required code out of the if statement for
'export', as it needs to be run for 'readonly' as well. This bugfix
is from https://github.com/att/ast/pull/906

src/cmd/ksh93/bltins/typeset.c:
- Set 'tdata.sh->prefix' to the correct value, otherwise 'readonly'
  uses the wrong scope.

src/cmd/ksh93/tests/builtins.sh:
- Add the regression test from ksh2020, modified to run in a
  subshell.

src/cmd/ksh93/data/builtins.c,
src/cmd/ksh93/sh.1:
- Add documentation of 'readonly' vs. 'typeset -r' difference:
  'readonly' does not create a function-local scope.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

src/cmd/ksh93/tests/builtins.sh

@@ -726,5 +726,17 @@ chmod is a tracked alias for $(whence -p chmod)"
 	cd ../.bar
 ) || err_exit 'cd ../.bar when ../.bar exists should not fail'
 
+# ======
+# 'readonly' should set the correct scope when creating variables in functions
+unset foo
+(
+	function test_func
+	{
+		readonly foo="bar"
+		[[ "$foo" = "bar" ]] || err_exit "readonly variable is not assigned a value inside functions"
+	}
+	test_func
+)
+
 # ======
 exit $((Errors<125?Errors:125))