Terminal scrollback with ring buffer

This patch adds a ring buffer for scrollback to the terminal.  The
advantage of using a ring buffer is that the common case, scrolling with
no static screen content, can be achieved very efficiently by
incrementing and decrementing the starting line (modulo buffer size).

The scrollback buffer is limited to HISTSIZE lines in order to bound
memory usage. As the lines are allocated on demand, it is possible to
implement unlimited scrollback with few changes.  If the terminal is
reset, the scroll back buffer is reset, too.

(apply st-scrollback-ringbuffer-0.8.5.diff on top of colorscheme)

x.c

@@ -61,6 +61,8 @@ static void zoomreset(const Arg *);
 static void ttysend(const Arg *);
 static void nextscheme(const Arg *);
 static void selectscheme(const Arg *);
+void kscrollup(const Arg *);
+void kscrolldown(const Arg *);
 
 /* config.h for applying patches and the configuration. */
 #include "config.h"