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/init.c

@@ -1805,13 +1805,9 @@ static Init_t *nv_init(void)
 	(OPTINDNOD)->nvalue.lp = (&sh.st.optindex);
 	/* set up the seconds clock */
 	sh.alias_tree = dtopen(&_Nvdisc,Dtoset);
-	dtuserdata(sh.alias_tree,&sh,1);
 	sh.track_tree = dtopen(&_Nvdisc,Dtset);
-	dtuserdata(sh.track_tree,&sh,1);
 	sh.bltin_tree = sh_inittree((const struct shtable2*)shtab_builtins);
-	dtuserdata(sh.bltin_tree,&sh,1);
 	sh.fun_base = sh.fun_tree = dtopen(&_Nvdisc,Dtoset);
-	dtuserdata(sh.fun_tree,&sh,1);
 	dtview(sh.fun_tree,sh.bltin_tree);
 	nv_mount(DOTSHNOD, "type", sh.typedict=dtopen(&_Nvdisc,Dtoset));
 	nv_adddisc(DOTSHNOD, shdiscnames, (Namval_t**)0);
@@ -1853,7 +1849,6 @@ Dt_t *sh_inittree(const struct shtable2 *name_vals)
 		nbltins = n;
 	}
 	base_treep = treep = dtopen(&_Nvdisc,Dtoset);
-	dtuserdata(treep,&sh,1);
 	treep->user = (void*)&sh;
 	for(tp=name_vals;*tp->sh_name;tp++,np++)
 	{