Windows virtual terminal sequences

Microsoft has been making a recommendation that programs should switch
from using the classic low-level console API calls to virtual terminal
sequences for a couple of years.

References:
"Our recommendation is to replace the classic Windows Console API with virtual
terminal sequences. This article will outline the difference between the two
and discuss the reasons for our recommendation."
From:
Classic Console APIs versus Virtual Terminal Sequences
https://docs.microsoft.com/en-us/windows/console/classic-vs-vt

The online documentation for WriteConsoleOutputCharacter() and
WriteConsoleOutputAttribute() have this disclaimer on them:
"This document describes console platform functionality that is no longer a
part of our ecosystem roadmap. We do not recommend using this content in new
products, but we will continue to support existing usages for the indefinite
future. Our preferred modern solution focuses on virtual terminal sequences
for maximum compatibility in cross-platform scenarios. You can find more
information about this design decision in our classic console vs. virtual
terminal document."

Since NetHack started out as a terminal program, before there was a
Windows "classic" console API introduced with Windows NT, it seemed only
fitting that the Windows console port should evolve in the virtual terminal
direction.

This is a first stab at it. The performance won't be as instantaneous as
the low-level console API's. That's likely partly because of this consoletty.c
initial implementation, but it may also partly be because under the hood in
the OS, there's recognitions/translations/conversions going on. Microsoft
states it will continue to evolve the Windows Terminal and console, and
hopefully it will improve. Hopefully it isn't too slow to play. It still
attempts to take advantage of the back buffer stuff that Barton House
introduced to minimize screen updates. At this point, it can still be
recompiled without the virtual terminal support by defining NO_VT when
compiling consoletty.c, or by commenting out the definition of
VIRTUAL_TERMINAL_SEQUENCES at the top of sys/windows/consoletty.c

That's the informational news, and the negative news out of the way.

There's some good news too. Because the virtual terminal sequences
support include 24-bit color support, the Windows console under virtual
terminal sequence can provide a more pleasant set of colors to the NetHack
console interface. To that end, some color changes have been implemented
in consoletty.c now.

It makes the console port ready to accept and display 24-bit color from
the NetHack core, if that should ever happen, as well.

As usual with a first implementation, there may be some bugs. Reports
are welcome.

src/symbols.c

@@ -15,7 +15,9 @@ void (*decgraphics_mode_callback)(void) = 0; /* set in tty_start_screen() */
 void (*ibmgraphics_mode_callback)(void) = 0; /* set in tty_start_screen() */
 void (*ascgraphics_mode_callback)(void) = 0; /* set in tty_start_screen() */
 #endif
-
+#ifdef WIN32
+void (*ibmgraphics_mode_callback)(void) = 0; /* set in tty_start_screen() */
+#endif
 #ifdef CURSES_GRAPHICS
 void (*cursesgraphics_mode_callback)(void) = 0;
 #endif