Fix nested functions ignoring prefixed variable assignments (#37)

This commit fixes the bug described in att/ast#32. The fix and
following explanation is from att/ast#467:

While copying variables from function's local scope to a new scope,
variable attributes were not copied. Such variables were not marked
to be exported in the new function. For e.g.

function f2 { env | grep -i "^foo"; }
function f1 { env | grep -i "^foo"; f2; }
foo=bar f1

prints 'foo=bar' only once, but it should print be twice.

src/cmd/ksh93/sh/xec.c:
 - When variables from the local scope of a function are copied into
   the scope of a nested function, the attributes of the variables
   need to be copied as well.

src/cmd/ksh93/tests/functions.sh:
 - Add regression tests from ksh2020 to check environment variables
   passed to functions.

NEWS

@@ -14,6 +14,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Fixed a bug that caused the kill and stop commands to segfault when given
   a non-existent job.
 
+- Nested functions no longer ignore variable assignments that were prefixed
+  to their parent function, i.e. 'VAR=foo func' will now set $VAR to 'foo'
+  in the scope of any nested function 'func' runs.
+
 2020-06-20:
 
 - Fixed a bug that caused setting the following variables as readonly in