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)

mac/argument.h

@@ -29,10 +29,19 @@ public:
     Argtype m_argtype; // {};
     bool m_optional {};
     std::string m_default {};
-    // True when m_default was filled from the argument type's :default
-    // rather than declared in the klammer's parameter list (for kdesc
-    // provenance display).
+    // Where m_default came from, for kdesc provenance display.  Default
+    // resolution has three levels -- the argument type's :default, an option
+    // set's declaration, and the site where the set is used (or the klammer's
+    // own parameter list) -- and a reader of kdesc is entitled to know which
+    // one produced the value a klammer will use when the argument is absent.
+    // True when m_default was filled from the argument type's :default:
     bool m_default_from_type {};
+    // The option set (".o" target) this parameter was declared in; empty when
+    // it was declared in the klammer's own parameter list.
+    std::string m_option_set {};
+    // True when the option set declares a default for this parameter and the
+    // declaration using the set overrode it.
+    bool m_default_overridden {};
     Locator m_loc;
     std::string m_target {};
 };