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/unixconf.h

@@ -302,8 +302,6 @@
 
 #if defined(BSD) || defined(ULTRIX)
 #include <sys/time.h>
-#else
-#include <time.h>
 #endif
 
 /* these might be needed for include/system.h;
@@ -324,10 +322,7 @@
 #define SHELL /* do not delete the '!' command */
 #endif
 
-/* #include "system.h" */
-
 #if defined(POSIX_TYPES) || defined(__GNUC__)
-#include <stdlib.h>
 #include <unistd.h>
 #endif