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/data/builtins.c

@@ -1360,7 +1360,7 @@ USAGE_LICENSE
 ;
 
 const char sh_optreadonly[] =
-"[-1c?\n@(#)$Id: readonly (AT&T Research) 2008-06-16 $\n]"
+"[-1c?\n@(#)$Id: readonly (AT&T Research/ksh93) 2020-06-28 $\n]"
 USAGE_LICENSE
 "[+NAME?readonly - set readonly attribute on variables]"
 "[+DESCRIPTION?\breadonly\b sets the readonly attribute on each of "
@@ -1368,6 +1368,9 @@ USAGE_LICENSE
 	"values from being changed.  If \b=\b\avalue\a is specified, "
 	"the variable \aname\a is set to \avalue\a before the variable "
 	"is made readonly.]"
+"[+?Unlike \btypeset -r\b, \breadonly\b does not create a function-local "
+	"scope and the given \aname\as are marked globally read-only by "
+	"default.]"
 "[+?Within a type definition, if the value is not specified, then a "
 	"value must be specified when creating each instance of the type "
         "and the value is readonly for each instance.]"