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/vms/vmsfiles.c

@@ -8,7 +8,6 @@
  *  routines or substitute for ones where we want behavior modification.
  */
 #include "config.h"
-#include <ctype.h>
 
 #ifdef VMSVSI
 #include <descrip.h>

sys/vms/vmsmail.c

@@ -20,7 +20,6 @@ struct mail_info *parse_next_broadcast(void);
 #ifdef MAIL
 #include "wintype.h"
 #include "winprocs.h"
-#include <ctype.h>
 #include <descrip.h>
 #include <errno.h>
 #ifndef __GNUC__

sys/vms/vmsunix.c

@@ -29,7 +29,6 @@
 #include <stat.h>
 #undef umask
 #endif
-#include <ctype.h>
 
 extern int debuggable; /* defined in vmsmisc.c */