src/config/rcxml.c: distinguish no and unknown font places

src/config/rcxml.c: distinguish no and unknown font places

Currently, the `rc.xml` parser applies font settings in a `<font>` tag with an
unknown value for its `place` attribute to all fonts. This means that whatever
the final unknown-`place` `<font>` tag is in a user's `rc.xml` applies to all
text drawn by labwc.

Instead, only treat `<font>` tags with an empty or missing `place` attribute as
applying globally, and warn when encountering unknown `place` attribute values
(which will help us find font places to support).