win32 updates (from <Someone>)

- restructured Install.nt quite a bit. It now contains instructions
to build a graphical nethack using NMAKE, too. I merged
the instructions for command line builds, and separated the IDE
build; that made more sense to me. It is shorter, too.
- added some lines to all Makefiles so they now build
NetHackW.exe when GRAPHICAL is "Y", and NetHack.exe
otherwise. I espacially did not test this on Borland.
Previously, the makefiles would always build NetHack.exe.
- changed the IDE files to build NetHackW.exe instead of
nethackw.exe. This is only cosmetic, but consistent with the
other executable.
- made a small change to pcmain.c, as the MinGW linker
cannot decide between main() and WinMain() when both are
present, as explained in <Someone>'s original
message. (I used a #ifndef instead of comments ;-)
The MinGW graphical build indeed seems to work.

sys/share/pcmain.c

@@ -80,6 +80,11 @@ extern unsigned _stklen;
 #endif
 
 #ifdef OVL0
+/* If the graphics version is built, we don't need a main; it is skipped
+ * to help MinGW decide which entry point to choose. If both main and 
+ * WinMain exist, the resulting executable won't work correctly.
+ */
+#ifndef MSWIN_GRAPHICS
 int
 main(argc,argv)
 int argc;
@@ -94,6 +99,7 @@ char *argv[];
      /*NOTREACHED*/
      return 0;
 }
+#endif /*MSWIN_GRAPHICS*/
 #endif /*OVL0*/
 #ifdef OVL1