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/lib/libast/Mamfile

@@ -359,7 +359,6 @@ make install
 					exec - 0) rm -f astmath.exe ;;
 					exec - *) touch astmath.exe ;;
 					exec - esac
-					exec - rm -f astmath
 				done astmath.exe dontcare generated
 				prev FEATURE/aso
 				exec - sed -e '/^#define _REQ_/!d' -e 's/#define _REQ_\([a-z0-9_]*\).*/ -l\1/' FEATURE/aso >> ast.req

src/lib/libast/man/stak.3

@@ -54,9 +54,9 @@ There is a preset initial active stack.
 To use an additional stack, it is necessary to create it and to
 install it as the active stack.
 A stack is created with the \f3stakcreate\fP() function.
-A \fIflags\fP argument of \f3STAK_SMALL\fP indicates that unused
-space on the stack should be freed whenever this stack ceases
-to be the active stack.
+The \fIflags\fP argument is an options bitmask.
+If the \f3STAK_SMALL\fP bit is set, the stack allocates memory in
+small blocks, optimizing for memory usage at the expense of performance.
 If successful,
 \f3stakcreate\fP() returns a pointer to a stack whose reference
 count is 1.

src/lib/libast/man/stk.3

@@ -55,9 +55,11 @@ There is a preset initial active stack.
 To use an additional stack, it is necessary to create it and to
 install it as the active stack.
 A stack is created with the \f3stkopen\fP() function.
-A \fIflags\fP argument of \f3STK_SMALL\fP indicates that unused
-space on the stack should be freed whenever this stack ceases
-to be the active stack.
+The \fIflags\fP argument is an options bitmask.
+If the \f3STK_SMALL\fP bit is set, the stack allocates memory in
+small blocks, optimizing for memory usage at the expense of performance.
+If the \f3STK_NULL\fP bit is set, a stack overflow will cause stack
+operations to return a null pointer instead of throwing an error.
 If successful,
 \f3stkopen\fP() returns a pointer to a stack whose reference
 count is 1.

src/lib/libast/misc/stk.c

@@ -229,11 +229,9 @@ Sfio_t *stkopen(int flags)
 	if(flags&STK_NULL) sp->stkoverflow = 0;
 	else sp->stkoverflow = stkcur?stkcur->stkoverflow:overflow;
 	bsize = init+sizeof(struct frame);
-#ifndef USE_REALLOC
 	if(flags&STK_SMALL)
 		bsize = roundof(bsize,STK_FSIZE/16);
 	else
-#endif /* USE_REALLOC */
 		bsize = roundof(bsize,STK_FSIZE);
 	bsize -= sizeof(struct frame);
 	if(!(fp=newof((char*)0,struct frame, 1,bsize)))
@@ -279,9 +277,6 @@ Sfio_t *stkinstall(Sfio_t *stream, _stk_overflow_ oflow)
 		if(stream!=stkstd)
 			sfstack(stkstd,stream);
 		stkcur = sp;
-#ifdef USE_REALLOC
-		/*** someday ***/
-#endif /* USE_REALLOC */
 	}
 	else
 		sp = stkcur;
@@ -509,10 +504,8 @@ static char *stkgrow(register Sfio_t *stream, size_t size)
 	int nn=0,add=1;
 	n += (m + sizeof(struct frame)+1);
 	if(sp->stkflags&STK_SMALL)
-#ifndef USE_REALLOC
 		n = roundof(n,STK_FSIZE/16);
 	else
-#endif /* !USE_REALLOC */
 		n = roundof(n,STK_FSIZE);
 	/* see whether current frame can be extended */
 	if(stkptr(stream,0)==sp->stkbase+sizeof(struct frame))