Fix set/unset state for short integer (typeset -si) (#211)

This commit fixes at least three bugs:
1. When issuing 'typeset -p' for unset variables typeset as short
   integer, a value of 0 was incorrectly diplayed.
2. ${x=y} and ${x:=y} were still broken for short integer types
   (re: 9f2389ed). ${x+set} and ${x:+nonempty} were also broken.
3. A memory fault could occur if typeset -l followed a -s option
   with integers. Additonally, now the last -s/-l wins out as the
   option to utilize instead of it always being short.

src/cmd/ksh93/include/name.h:
- Fix the nv_isnull() macro by removing the direct exclusion of
  short integers from this set/unset test. This breaks few things
  (only ${.sh.subshell} and ${.sh.level}, as far as we can tell)
  while potentially correcting many aspects of short integer use
  (at least bugs 1 and 2 above), as this macro is widely used.
- union Value: add new pid_t *pidp pointer member for PID values
  (see further below).

src/cmd/ksh93/bltins/typeset.c: b_typeset():
- To fix bug 3 above, unset the 'shortint' flag and NV_SHORT
  attribute bit upon encountering the -l optiobn.

*** To fix ${.sh.subshell} to work with the new nv_isnull():

src/cmd/ksh93/sh/defs.h:
- Add new 'realsubshell' member to the shgd (aka shp->gd) struct
  which will be the integer value for ${.sh.subshell}.

src/cmd/ksh93/sh/init.c,
src/cmd/ksh93/data/variables.c:
- Initialize SH_SUBSHELLNOD as a pointer to shgd->realsubshell
  instead of using a short value (.s) directly. Using a pointer
  allows nv_isnull() to return a positive for ${.sh.subshell} as
  a non-null pointer is what it checks for.
- While we're at it, initialize PPIDNOD ($PPID) and SH_PIDNOD
  (${.sh.pid}) using the new pdip union member, which is more
  correct as they are values of type pid_t.

src/cmd/ksh93/sh/subshell.c,
src/cmd/ksh93/sh/xec.c:
- Update the ${.sh.subshell} increases/decreases to refer to
  shgd->realsubshell (a.k.a. shp->gd->realsubshell).

*** To fix ${.sh.level} after changing nv_isnull():

src/cmd/ksh93/sh/macro.c: varsub():
- Add a specific exception for SH_LEVLNOD to the nv_isnull() test,
  so that ${.sh.level} is always considered to be set. Its handling
  throughout the code is too complex/special for a simple fix, so
  we have to special-case it, at least for now.

*** Regression test additions:

src/cmd/ksh93/tests/attributes.sh:
- Add in missing short integer tests and correct the one that
  existed. The -si test now yields 'typeset -x -r -s -i foo'
  instead of 'typeset -x -r -s -i foo=0' which brings it in line
  with all the others.
- Add in some other -l attribute tests for floats. Note, -lX test
  was not added as the size of long double is platform dependent.

src/cmd/ksh93/tests/variables.sh:
- Add tests for ${x=y} and ${x:=y} used on short int variables.

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

src/cmd/ksh93/sh/macro.c

@@ -1409,7 +1409,7 @@ retry1:
 		if(np && type==M_BRACE && mp->shp->argaddr)
 			nv_optimize(np);  /* needed before calling nv_isnull() */
 #endif /* SHOPT_OPTIMIZE */
-		if(np && (type==M_BRACE ? !nv_isnull(np) : (type==M_TREE || !c || !ap)))
+		if(np && (type==M_BRACE ? (!nv_isnull(np) || np==SH_LEVELNOD) : (type==M_TREE || !c || !ap)))
 		{
 			/* Either the parameter is set, or it's a special type of expansion where 'unset' doesn't apply. */
 			char *savptr;