Update xdot description;
note that dot or xdot output may create attributes that affect future
layouts
Add declaration for textfont_dict_close.
Update libguide to note that a program needs to link in the cgraph library,
not the graph library.
Use explicit _background attribute for specifying background graphics
in xdot, rather than just using the graph's _draw_ attribute. This
avoids the problem of having the _draw_ attribute growing each time
_draw_ is defined and the graph is rendered.
Fix github issues/3
Replace AI_PASSIVE flag with AI_ADDRCONFIG | AI_NUMERICSERV