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/fhandler.h

@@ -1778,6 +1778,8 @@ enum ansi_intensity
 #define eattitle 7
 #define gotparen 8
 #define gotrparen 9
+#define eatpalette 10
+#define endpalette 11
 #define MAXARGS 10
 
 enum cltype
@@ -1791,6 +1793,8 @@ enum cltype
 
 class dev_console
 {
+  pid_t owner;
+
   WORD default_color, underline_color, dim_color;
 
   /* Used to determine if an input keystroke should be modified with META. */