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_grid.py

@@ -20,7 +20,13 @@ class Kargs:
         self.caption_side_center = False
         self.Image_search_path = K.Image_search_path
         self.number = K.cell_number
+        # A cell is placed by the grid, not by itself: no positioning
+        # container, and nothing to inset it from.  Both members of the
+        # hpos_args option set have to be set here -- image.py reads them
+        # unconditionally, so a missing one is an AttributeError at render
+        # time rather than a message.
         self.hpos = "none"
+        self.offset = "0pt"
         self.K_target = target
         self.K_input_dir = K.K_input_dir
         self.K_output_dir = K.K_output_dir