Cygwin: console: support 24 bit color

- Add 24 bit color support using xterm compatibility mode in
  Windows 10 1703 or later.
- Add fake 24 bit color support for legacy console, which uses
  the nearest color from 16 system colors.

winsup/cygwin/environ.cc

@@ -869,7 +869,8 @@ win32env_to_cygenv (PWCHAR rawenv, bool posify)
   char *newp;
   int i;
   int sawTERM = 0;
-  static char NO_COPY cygterm[] = "TERM=cygwin";
+  const char cygterm[] = "TERM=cygwin";
+  const char xterm[] = "TERM=xterm-256color";
   char *tmpbuf = tp.t_get ();
   PWCHAR w;
 
@@ -899,8 +900,10 @@ win32env_to_cygenv (PWCHAR rawenv, bool posify)
       debug_printf ("%p: %s", envp[i], envp[i]);
     }
 
+  /* If console has 24 bit color capability, TERM=xterm-256color,
+     otherwise, TERM=cygwin */
   if (!sawTERM)
-    envp[i++] = strdup (cygterm);
+    envp[i++] = strdup (wincap.has_con_24bit_colors () ? xterm : cygterm);
 
   envp[i] = NULL;
   return envp;