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

win/curses/cursmesg.c

@@ -8,7 +8,6 @@
 #include "wincurs.h"
 #include "cursmesg.h"
 #include "curswins.h"
-#include <ctype.h>
 
 /* defined in sys/<foo>/<foo>tty.c or cursmain.c as last resort;
    set up by curses_init_nhwindows() */