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)

com/kdesc.cpp

@@ -101,6 +101,7 @@ int main(int argc, char* argv[])
         args.opt("input", "Input filename", "filename", "", "'text'");
         args.flag("targets", "Show targets defined by the input file");
         args.flag("klammers", "Show klammers defined by the input file");
+        args.flag("optionsets", "Show option sets declared by the input file");
         args.var("font", "List installed fonts.  Enter \"--font help\" for font maintenance commands.");
         args.var("klammerset", "List the klammersets on the search path.  Enter \"--klammerset help\" for details.");
         args.opt("v",  "'verbosity'", "n", "0", "'verbosity'");
@@ -190,6 +191,10 @@ int main(int argc, char* argv[])
             std::cout << boldblack << "Klammers\n" << black << M.m_klammers.describe(2);
         }
 
+        if (p("optionsets")) {
+            std::cout << boldblack << "Option sets\n" << black << M.m_option_sets.describe(2);
+        }
+
     }
     catch (Error& e) {
         e.print_message();