Another round of accumulated tweaks and cleanups

Notable changes:

src/cmd/ksh93/*.c:
- Get rid of all the dtuserdata(FOO,&sh,1) calls backported in
  cc492752. These set pointers to sh in Cdt objects. As of
  b590a9f1, the code does not use any pointers to sh, so these are
  superfluous.

src/cmd/ksh93/data/builtins.c,
src/cmd/ksh93/sh.1:
- As of ksh 93l 2001-06-01, the -h/trackall option has no effect at
  all, so trim its documentation.

src/lib/libast/man/stk.3,
src/lib/libast/man/stak.3:
- Correct the documentation on what the ST(A)K_SMALL option bit
  actually does based on a reading of the code.
- Document the STK_NULL option bit.

README.md,
src/cmd/ksh93/README:
- Add a note that -fdiagnostics-color=always will break the build.
  Ref.: https://github.com/ksh93/ksh/issues/379

src/lib/libast/Mamfile:
- Remove a 'rm -f astmath' command -- a file that is never created.
  But on Cygwin this removes astmath.exe, which *is* used. As a
  result, executing it failed on Cygwin, so the system incorrectly
  detected that Cygwin needs -lm for math functions.

src/cmd/ksh93/sh/arith.c

@@ -180,10 +180,7 @@ static Namval_t *scope(register Namval_t *np,register struct lval *lvalue,int as
 			{
 				ap = nv_arrayptr(np);
 				if(ap && !ap->table)
-				{
 					ap->table = dtopen(&_Nvdisc,Dtoset);
-					dtuserdata(ap->table,&sh,1);
-				}
 				if(ap && ap->table && (nq=nv_search(nv_getsub(np),ap->table,NV_ADD)))
 					nq->nvenv = (char*)np;
 				if(nq && nv_isnull(nq))