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/table/table.k

@@ -193,10 +193,10 @@
 
 @@rowcolor.tex s : \colorrow{*s*} @@
 
-@@table rows.rest(2)
+@@table.k rows.rest(2)
   :id
-  @caption_arguments@
-  :hpos.element_hpos
+  @caption_args@
+  @hpos_args@
   :header.bool true
   :allow_break.bool false
   :column_width.column_width
@@ -215,9 +215,20 @@
   :leading.float 1.3
   :colsep 4pt
   :
-  @eval table.Table(K) eval@
+A table of rows of cells.  The cells of a row are separated by "|" and the
+rows by "||", so the argument is two-dimensional; the first row is the
+header unless ^:header is false.
+
+Everything else is optional, and a table written with no options is a plain
+grid of its cells.  ^:column_width lays the columns out, ^:hline and ^:vline
+draw lines, ^:colspan and ^:rowspan merge cells, ^:justify and ^:cell_hpos
+place text within them, ^:calc computes cells from other cells and ^:format
+formats them, and ^:hpos and ^:offset place the whole table in the text
+column.
 @@
 
+@@table :: @eval table.Table(K) eval@ @@
+
 @@tbl spec.figure_id :
   @reference *spec* | Table @
 @@