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.py

@@ -103,7 +103,8 @@ class Image(klammer_base.Klammer_base):
 
         result = html_util.add_caption(
             result, "Figure", self.number, self.caption, self.caption_font,
-            self.hpos, self.caption_side, False, self.caption_font_size)
+            self.hpos, self.caption_side, False, self.caption_font_size,
+            offset=self.offset)
 
         result = E("div").body(result).cls("image_margin")
 
@@ -131,16 +132,19 @@ class Image(klammer_base.Klammer_base):
                 self.caption += self.file_error_message
             result = latex_util.add_caption(
                 result, "Figure", self.number, self.caption, width,
-                self.hpos, self.caption_side)
+                self.hpos, self.caption_side, offset=self.offset)
                 #result, "Figure", self.number, self.caption, self.hpos, self.caption_side,
                 #self.width, self.caption_side_center, self.vmargin, self.caption_margin)
         else:
             result = f'\\includegraphics[width={width}]{{{source}}}'
-            result = latex_util.caption_wrapper(result, self.hpos)
+            result = latex_util.caption_wrapper(result, self.hpos, offset=self.offset)
         name = f"Reference-Figure-{Image.id}"
         result = f"\\hypertarget{{{name}}}{{}}\\label{{Label-{name}}}\n{result}"
         Image.id += 1
-        return result
+        # \includegraphics (bare or inside a caption wrapper's minipage) is
+        # box material: it must sit in vertical mode or it is typeset beside
+        # any text it follows.
+        return latex_util.block(result)
 
 
     def txt(self):