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-pre2.370

@@ -177,10 +177,12 @@ MSDOS_GPP_CFLAGS = -Wall -Wextra -Wno-missing-field-initializers -Wreturn-type \
 MSDOS_TARGET_CFLAGS = -c -O $(DBGFLAGS) -I../include -I../sys/msdos -I../win/share \
 	$(LUAINCL) -DDLB $(PDCURSESDEF) -DTILES_IN_GLYPHMAP \
 	-DCROSSCOMPILE -DCROSSCOMPILE_TARGET -DCROSS_TO_MSDOS \
+	-D_NAIVE_DOS_REGS \
 	$(MSDOS_GCC_CFLAGS)
 MSDOS_TARGET_CXXFLAGS = -c -O $(DBGFLAGS) -I../include -I../sys/msdos -I../win/share \
         $(LUAINCL) -DDLB $(PDCURSESDEF) \
         -DUSE_TILES -DCROSSCOMPILE -DCROSSCOMPILE_TARGET -DCROSS_TO_MSDOS \
+	-D_NAIVE_DOS_REGS \
 	$(MSDOS_GPP_CFLAGS)
 PDCINCL += -I$(PDCPORT)
 PDC_TARGET_CFLAGS = $(MSDOS_TARGET_CFLAGS) -Wno-unused-parameter \