curses follow-up bits

Move the curses global variable defininitions to cursmain.c.

Make the references to those global variables extern in
include/wincurs.h

Get rid of a warning:
../win/curses/cursmesg.c:379:9: warning: declaration shadows a
variable in the global scope [-Wshadow] int orig_cursor = curs_set(0);

Kludge for Visual Studio compiler: Add a stub- file for use
in Windows curses port builds to ensure that a needed #pragma
is invoked prior to compiling the file pdcscrn.c in the
PDCurses source distribution. All command line options and
compile of the file. It is unreasonable to expect a NetHack
builder to have to tinker with the PDCurses source files in
order to build NetHack. This kludge means the NetHack builder
doesn't have to.
The file stub-pdcscrn.c contains only two lines:
	#pragma warning(disable : 4996)
	#include "pdcscrn.c"
Some day, if the PDCurses sources corrects the issue, this
can go away.

include/wincurs.h

@@ -7,16 +7,11 @@
 
 /* Global declarations for curses interface */
 
-int term_rows, term_cols; /* size of underlying terminal */
-
-WINDOW *base_term;    /* underlying terminal window */
-
-WINDOW *mapwin, *statuswin, *messagewin;    /* Main windows */
-
-int orig_cursor;	/* Preserve initial cursor state */
-
-boolean counting;   /* Count window is active */
-
+extern int term_rows, term_cols;   /* size of underlying terminal   */
+extern int orig_cursor; 	   /* Preserve initial cursor state */
+extern WINDOW *base_term;          /* underlying terminal window    */
+extern boolean counting;           /* Count window is active        */
+extern WINDOW *mapwin, *statuswin, *messagewin;    /* Main windows  */
 
 #define TEXTCOLOR   /* Allow color */
 #define NHW_END 19