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

include/windconf.h

@@ -165,8 +165,6 @@ typedef SSIZE_T ssize_t;
 
 
 #include <sys/types.h>
-#include <stdlib.h>
-#include <stdio.h>
 #ifdef __BORLANDC__
 #undef randomize
 #undef random
@@ -185,10 +183,6 @@ typedef SSIZE_T ssize_t;
 #endif
 
 #define NO_SIGNAL
-
-/* Time stuff */
-#include <time.h>
-
 #define USE_STDARG
 
 /* Use the high quality random number routines. */