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/unix/hints/include/cross-post.370

@@ -79,8 +79,14 @@ ifeq "$(WANT_DEBUG)" "1"
 		pwd; echo "$(GDBEXE) not found and WANT_DEBUG=1 specified"; \
 	  fi; )
 else
-	-rm $(TARGETPFX)pkg/GDB.EXE
-	-rm $(TARGETPFX)pkg/NHGDB.BAT
+	-( if [ -f $(TARGETPFX)pkg/GDB.EXE ]; \
+	  then \
+		rm $(TARGETPFX)pkg/GDB.EXE; \
+	  fi; )
+	-( if [ -f $(TARGETPFX)pkg/NHGDB.BAT ]; \
+	  then \
+		rm $(TARGETPFX)pkg/NHGDB.BAT; \
+	  fi; )
 endif
 	-touch $(TARGETPFX)pkg/RECORD
 	cd $(TARGETPFX)pkg ; zip -9 ../NH370DOS.ZIP * ; cd ../../..