more C standard progress

There was a transcription error in the comments in cstd.h for
the standard list of header files, where only the description
remained for <stdlib.h>, not the name of the file itself.

Remove several extraneous inclusions of the standard C99 headers.

Tested on the following afterwards:
Linux (using hints/linux.370) including tty, curses, qt6, and X11
macOS (using hints/macOS.370) including tty, curses, qt5, and X11
Windows MSYS2 using sys/windows/GNUmakefile
Windows Visual Studio using sys/windows/Makefile.nmake
msdos cross-compile on Ubuntu using djgpp cross-compiler

sys/msdos/Makefile.GCC

@@ -458,18 +458,11 @@ endif
 INCLDIR=$(PDCINCL) -I../include -I../sys/msdos $(LUAINCL)
 
 # Debugging
-#cflags = -pg -c $(INCLDIR) $(DLBFLG) $(CURSESDEF) -DSUPPRESS_GRAPHICS
+#cflags = -pg -c -D_NAIVE_DOS_REGS $(INCLDIR) $(DLBFLG) $(CURSESDEF) -DSUPPRESS_GRAPHICS
 #LFLAGS = -pg
 
-cflags = -c -O $(INCLDIR) $(DLBFLG) $(CURSESDEF) -DSUPPRESS_GRAPHICS
-LFLAGS =
-
-# Debugging
-#cflags = -g -c $(INCLDIR) $(DLBFLG) $(CURSESDEF) -DTILES_IN_GLYPHMAP
-#LFLAGS = -g
-
 #    Normal
-cflags = -c -O $(INCLDIR) $(DLBFLG) $(CURSESDEF) -DTILES_IN_GLYPHMAP
+cflags = -c -O -D_NAIVE_DOS_REGS $(INCLDIR) $(DLBFLG) $(CURSESDEF) -DTILES_IN_GLYPHMAP
 LFLAGS =
 
 #==========================================

sys/msdos/msdos.c

@@ -15,7 +15,6 @@
 #include "pcvideo.h"
 
 #include <dos.h>
-#include <ctype.h>
 
 /*
  * MS-DOS functions

sys/msdos/tile2bin.c

@@ -18,10 +18,6 @@
 #include "pctiles.h"
 
 /* #include <dos.h> */
-#ifndef MONITOR_HEAP
-#include <stdlib.h>
-#endif
-#include <time.h>
 
 #ifdef __GO32__
 #include <unistd.h>

sys/msdos/video.c

@@ -105,7 +105,6 @@ void g_pututf8(uint8 *utf8str)
 
 #ifdef NO_TERMS
 
-#include <ctype.h>
 #include "wintty.h"
 
 #ifdef __GO32__

sys/msdos/vidtxt.c

@@ -18,7 +18,6 @@
 #include "wintty.h"
 
 #include <dos.h>
-#include <ctype.h>
 
 #if defined(_MSC_VER)
 #if _MSC_VER >= 700

sys/msdos/vidvga.c

@@ -14,7 +14,6 @@
 #include "font.h"
 
 #include <dos.h>
-#include <ctype.h>
 #include "wintty.h"
 
 #ifdef __GO32__