config: add ‘beam-thickness’ option

* Rename cursor.style value ‘bar’ to ‘beam’. ‘bar’ remains recognized,
  but should eventually be deprecated and then removed.
* Add ‘cursor.beam-thickness’ option, a pt-or-px value specifying the
  thickness of the beam cursor. Defaults to 1.5pt.
* Rename (and export) pt_or_px_as_pixels() to
  term_pt_or_px_as_pixels()
* Change term_pt_or_px_as_pixels() to round point values instead of
  truncating them.

config.h

@@ -154,6 +154,7 @@ struct config {
             uint32_t text;
             uint32_t cursor;
         } color;
+        struct pt_or_px beam_thickness;
     } cursor;
 
     struct {