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/show.cpp

@@ -570,6 +570,7 @@ std::ostream& operator<<(std::ostream& os, const Machine& m)
     std::string state_desc = m.m_state.describe(false, 3);
     std::string targets_desc = m.m_targets.describe(4);
     std::string klammersets_desc = m.m_klammersets.describe(4);
+    std::string option_sets_desc = m.m_option_sets.describe(4);
     std::string klammer_desc = m.m_klammers.describe(2);
     std::string source_desc = describe_sources(m);
 
@@ -578,6 +579,7 @@ std::ostream& operator<<(std::ostream& os, const Machine& m)
        << label("Argtypes", m.m_argtypes.m_names.size()) << argtypes_desc << "\n"
        << label("Targets", m.m_targets.m_names.size()) << targets_desc << "\n"
        << label("Klammersets", m.m_klammersets.m_symbols.size()) << klammersets_desc << "\n"
+       << label("Option sets", m.m_option_sets.m_names.size()) << option_sets_desc << "\n"
        << label("Klammers", m.m_klammers.m_klammers.size()) << klammer_desc << "\n"
        << label("State", m.m_state.m_frames.size()) << state_desc;
     return os;