Option sets: a .o target for shared parameters

A named group of optional parameters, declared once and used by several
klammers, so a writer learns one vocabulary instead of a spelling per
klammer.  The "o" target is a pseudo-target beside "k": "k" declares a
klammer's interface and documents it, "o" declares an option interface and
documents it, and neither produces output for any target.

    @@caption_args.o :caption :number.bool true :caption_side.side
    : Arguments that define a caption for a block element @@

    @@code.k :filename @hpos_args :hpos left @ @caption_args :caption_side top @
    | text.literal : A source file displayed verbatim @@

A set is used only in the parameter list of a ".k" declaration -- the one
place a klammer's interface is declared once for all of its targets -- and
is resolved as that list is read.  Names and types come from the set; a
default may be overridden where it is used.  A klammer application in a
parameter list is now a definition-time error.

The SKS gains the sets caption_args and hpos_args (:hpos and :offset), and
@table, @image, @image_grid, @reference and @show gain .k declarations.  A
distance is no longer written as a position: :hpos 4em is rejected, and the
same layout is :hpos left :offset 4em.  Code listings are numbered by
default, like tables and figures.

New engine sources mac/option_set{,_registry}.{h,cpp}; tst/ ships two more
suites, option_set_test.sh and signature_test.sh (twelve in all).

 (from dev 34e536cb0329)

sks/image/image.k

@@ -17,51 +17,45 @@
 # @@image.k basename : Image read from a file @@
 # @@image.html basename | width | height : @eval import image ; result = image.image(K) eval@ @@
 
-@@@argtype image_hpos |
-  horizontal position of an image: the element positions (center, left,
-  right, or a length used as the left margin), or none for an inline image
-  with no positioning container
-  :pattern 'element_hpos'^|none
-  :default center
-@@@
+# An image once had its own position type, for the sake of the "none" value
+# (no positioning container).  That value is meaningful for any block
+# element, so the shared hpos type carries it and an image is placed by the
+# hpos_args option set like every other block element.
 
-@@image
+@@image.k
   basename
   :id
   :width.length .5w
-  @caption_arguments@
+  @caption_args@
   :vmargin.bool true
-  :hpos.image_hpos
+  @hpos_args@
   :rel
   :abswidth.number 0.0
   :border.bool false
   :
-  @eval image.Image(K) eval@
+An image read from a file.  The basename names the file, which is looked
+for in the image search path and converted to a format the target can use.
+^:width is the image's width in the text column, ^:hpos and ^:offset place
+it there, and the caption arguments caption it.
 @@
 
-@@image_grid
+@@image :: @eval image.Image(K) eval@ @@
+
+@@image_grid.k
   image_specs.rest(2)
-  :caption
-  :number.bool true
   :cell_number.bool false
   :landscape.bool false
   :scale.number 0.98
-  :caption_side.caption_side
-  :caption_side_center.bool true
-#
+  @caption_args@
+  @hpos_args@
   :thumbnail.bool false
   :allow_break.bool true
-#  :indent.length
-#  :xmargin.length
   :id
-  :captionfont
-  :caption_width.float .9
   :rel
   :hsep.number 0.02
-  :
-  @eval image_grid.Image_grid(K) eval@
+: A grid of images
 @@
 
-@@fig spec.figure_id :
-  @reference *spec* | Figure @
-@@
+@@image_grid :: @eval image_grid.Image_grid(K) eval@ @@
+
+@@fig spec.figure_id : @reference *spec* | Figure @ @@