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/vmsconf.h

@@ -213,7 +213,6 @@ PANICTRACE_GDB=2  #at conclusion of panic, show a call traceback and then
 #include <types.h>
 #include <unistd.h>
 #include <fcntl.h>
-#include <stdlib.h>
 #include <signal.h>
 #include <stat.h>
 #include <errno.h>
@@ -246,8 +245,6 @@ typedef int32_t off_t;
 #endif
 #endif /* _DECC_V4_SOURCE */
 
-#include <time.h>
-
 #ifndef VMSVSI
 #if 0 /* <file.h> is missing for old gcc versions; skip it to save time */
 #include <file.h>