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/pcconf.h

@@ -111,33 +111,21 @@
 
 #if defined(_MSC_VER) && (_MSC_VER >= 7)
 #include <sys/types.h>
-#include <stdlib.h>
 #ifdef strcmpi
 #undef strcmpi
 #endif
-#include <string.h> /* Provides prototypes of strncmpi(), etc.     */
 #include <conio.h>
 #include <io.h>
 #include <direct.h>
 #define SIG_RET_TYPE void(__cdecl *)(int)
-#define M(c) ((char) (0x80 | (c)))
 #define vprintf printf
 #define vfprintf fprintf
 #define vsprintf sprintf
 #endif
 
-#if defined(__BORLANDC__) && defined(STRNCMPI)
-#include <string.h> /* Provides prototypes of strncmpi(), etc.     */
-#endif
-
-#if defined(__DJGPP__)
-#define _NAIVE_DOS_REGS
-#include <stdlib.h>
-#include <string.h> /* Provides prototypes of strncmpi(), etc.     */
 #ifndef M
 #define M(c) ((char) (0x80 | (c)))
 #endif
-#endif
 
 /*
  * On the VMS and unix, this option controls whether a delay is done by
@@ -226,10 +214,6 @@
 #define HLOCK "NHPERM"
 #endif
 
-#ifndef AMIGA
-#include <time.h>
-#endif
-
 /* the high quality random number routines */
 #ifndef USE_ISAAC64
 # ifdef RANDOM