Fix bugs in testing if a parameter is set

This fixes three related bugs:
1. Expansions like ${var+set} remained static when used within a
   'for', 'while' or 'until' loop; the expansions din't change
   along with the state of the variable, so they could not be used
   to check whether a variable is set within a loop if the state of
   that variable changed in the course of the loop. (BUG_ISSETLOOP)
2. ${IFS+s} always yielded 's', and [[ -v IFS ]] always yielded
   true, even if IFS is unset. (BUG_IFSISSET)
3. IFS was incorrectly exempt from '-u' ('-o nounset') checks.

src/cmd/ksh93/sh/macro.c: varsub():
- When getting a node pointer (np) to the parameter to test,
  special-case IFS by checking if it has a value and not setting
  the pointer if not. The node to IFS always exists, even after
  'unset -v IFS', so before this fix it always followed the code
  path for a parameter that is set. This fixes BUG_IFSISSET for
  ${IFS+s} and also fixes set -u (-o nounset) with IFS.
- Before using the 'nv_isnull' macro to check if a regular variable
  is set, call nv_optimize() if needed. This fixes BUG_ISSETLOOP.
  Idea from Kurtis Rader: https://github.com/att/ast/issues/1090
  Of course this only works if SHOPT_OPTIMIZE==1 (the default),
  but if not, then this bug is not triggered in the first place.
- Add some comments for future reference.

src/cmd/ksh93/bltins/test.c: test_unop():
- Fix BUG_IFSISSET for [[ -v IFS ]]. The nv_optimize() method
  doesn't seem to have any effect here, so the only way that I can
  figure out is to special-case IFS, nv_getval()'ing it to check if
  IFS has a value in the current scope.

src/cmd/ksh93/tests/variables.sh:
- Add regression tests for checking if a varariable is set within a
  loop, within and outside a function with that variable made local
  (to check if the scope is honoured). Repeat these tests for a
  regular variable and for IFS, for ${foo+set} and [[ -v foo ]].

(cherry picked from commit a2cf79cb98fa3e47eca85d9049d1d831636c9b16)

TODO

@@ -65,15 +65,6 @@ https://github.com/modernish/modernish/tree/0.16/lib/modernish/cap/
   are erroneously interpreted as wildcards when quoted "$*" is used as the
   glob pattern.
 
-- BUG_IFSISSET: ${IFS+s} always yields 's' even if IFS is unset. This applies
-  to IFS only.
-
-- BUG_ISSETLOOP: Expansions like ${var+set} remain static when used within a
-  'for', 'while' or 'until' loop; the expansions don't change along with the
-  state of the variable, so they cannot be used to check whether a variable
-  is set within a loop if the state of that variable may change in the
-  course of the loop.
-
 - BUG_KUNSETIFS: ksh93: Can't unset IFS under very specific circumstances.
   unset -v IFS is a known POSIX shell idiom to activate default field
   splitting. With this bug, the unset builtin silently fails to unset IFS