Fix up underscore usage.

In particular, stick to the documented pattern: all liblx-exported
things begin lx_ or LX_, with lx__ and LX__ reserved for internal
things which have to be experted anyway, such as the converters for
varargs arguments.  This mostly means inserting underscores, but in one
case (lx__priv -> lx_priv in LX_FONTINFO) it meant deleting one.