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.

foot.ini

@@ -42,6 +42,7 @@
 # style=block
 # color=111111 dcdccc
 # blink=no
+# beam-thickness=1.5
 
 [mouse]
 # hide-when-typing=no