Add scrollback buffer

Default keyboard shortcuts:
{ ShiftMask,		XK_Page_Up,	kscrollup,	{.i = -1} },
{ ShiftMask,		XK_Page_Down,	kscrolldown,	{.i = -1} },

Negative numbers scroll back by $LINES - i
Merge branch 'sb'

Conflicts:
	config.def.h
	st.c