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>

NEWS

@@ -11,6 +11,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Unsetting an array that was turned into a compound variable will no
   longer cause silent memory corruption.
 
+- Variables created with 'readonly' in functions are now set to the
+  specified value instead of nothing. Note that 'readonly' does not
+  create a function-local scope, unlike 'typeset -r' which does.
+
 2020-06-26:
 
 - Changing to a directory that has a name starting with a '.' will no