code-golfing: cleanup osc color related code

* adds missing function prototype
* move xgetcolor() prototype to win.h (that's where all the other x.c
  func prototype seems to be declared at)
* check for snprintf error/truncation
* reduces code duplication for osc 10/11/12
* unify osc_color_response() and osc4_color_response() into a single function

the latter two was suggested by Quentin Rameau in his patch review on
the hackers list.

win.h

@@ -30,6 +30,7 @@ void xdrawline(Line, int, int, int);
 void xfinishdraw(void);
 void xloadcols(void);
 int xsetcolorname(int, const char *);
+int xgetcolor(int, unsigned char *, unsigned char *, unsigned char *);
 void xseticontitle(char *);
 void xsettitle(char *);
 int xsetcursor(int);