Reduce visibility wherever possible

When possible, declare functions/variables static and move struct
definitions out of headers.  In order to allow utf8decode to become
internal, use codepoint for DECSCUSR extension directly.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

win.h

@@ -27,7 +27,6 @@ void xbell(void);
 void xclipcopy(void);
 void xdrawcursor(int, int, Glyph, int, int, Glyph);
 void xdrawline(Line, int, int, int);
-void xhints(void);
 void xfinishdraw(void);
 void xloadcols(void);
 int xsetcolorname(int, const char *);