Target coverage: a klammer states the targets it serves

kdesc gains --coverage, which reports for every klammer the set of targets it
can render to, and — the point of it — which klammers' coverage cannot be
derived and must therefore be declared.  Three rules: coverage is DERIVED
where the definitions determine it (a general body of klammer calls covers
the intersection of what those klammers cover, by a greatest fixpoint after
loading), DECLARED where the engine cannot interpret what decides it (an
@eval body, whose targets are undecidable), and UNKNOWN where nothing is
written — which never means "deliberately unavailable".

Two new spellings in a definition's name.  A comma-separated target list,
"@@table.html,tex :: ...", gives one body several targets; it is surface
syntax, expanded at registration, and each member goes through the
redefinition rules on its own.  And "@@date.* :: ..." writes the general
target out, asserting that the klammer works for EVERY target including ones
not yet defined — a stronger claim than a list of the targets defined today,
and the one target declaration that could be mechanically falsified.

The Standard Klammer Set was swept accordingly: it now has no general
definitions at all, every klammer names the targets it serves, six use ".*",
and tex and pdf are at zero undecided.

kdesc's flags are reorganised on two rules: a flag reached for often gets a
single letter (-k klammers, -t targets, -c characters, -i input), a more
specialised topic a multi-letter name (--argtypes, --katoms, --rewrite,
--optionsets, --coverage, --klammerset, --font); and -v says how much to show
about PROCESSING, never what the RESULT contains — so the katom regex column
is "--katoms full" and the coverage detail "--coverage all".  NOTE: "-k" now
lists klammers (optionally filtered by a name/description search); the katom
table moved to "--katoms".

Fixes carried along: an option written with no value crashed the command with
SIGSEGV instead of reporting the mistake; two required positional arguments
never parsed; kdesc and kdiag printed an error and exited 0; and definition
diagnostics counted registrations rather than what was written, so one line
could be reported as two definitions and then printed twice.

Four new test suites: target_list, coverage, command_option, kdesc.

(from dev 46f54080bd9a)

sks/font/font.k

@@ -58,35 +58,6 @@
 @@ri.tex :: \textrm{\textit{*text*}} @@
 @@ri.txt :: *text* @@
 
-
-#[
-@@small.k s :mode span : Smaller font size @@
-@@small.html  :: < #- *mode* class="small"> #- *s* #- </ #- *mode* #- > @@
-@@small.tex :: {\small *s*} @@
-
-@@large.k s :mode span : Larger font size @@
-@@large.html  :: <*mode* class="large">*s*</*mode*> @@
-@@large.tex :: {\Large *s*} @@
-
-@@rfont.k size | leading | text :  Scaled roman font @@
-
-@@rfont.tex ::
-%{\fontsize{*size*\basefontsize}{*leading* \basefontsize * \real{1.4}}\rmfamily
-{\fontsize{ *size* }{ *leading* * \real{1.4}}\rmfamily
-
-*text*
-
-}
-@@
-
-@@sfont.k size | leading | text :  Scaled sans-serif font @@
-@@sfont.tex ::
-{\fontsize{ *size* \basefontsize}{ *leading* \basefontsize * \real{1.4}}\sffamily
-*text*}
-@@
-]#
-
-
 @@ts.k scale.float | text : Resize *text* by *scale* @@
 # @@ts.tex :: {\scalefont{*scale*}\em *text*} @@
 @@ts.tex :: \scalefont{*scale*}{*text*} @@
@@ -97,37 +68,32 @@
 #@@leading.tex :: \renewcommand{\baselinestretch}{*n*} @@
 @@leading.tex :: \setstretch{*n*} @@
 
-# @@default_font : @font EB Garamond @ @@
-# @@default_font : @font Bitstream Charter @ @@
-@@default_font :
+@@default_font.k : Default font for documents @@
+@@default_font.tex ::
 # \setmainfont[Ligatures=TeX,Numbers=Lining]{Garamond Libre}
 \setmainfont{Garamond Libre}
 @@
-#[
-@font
-# Bitstream Charter
-# Liberation Serif
-EB Garamond
-@
-]#
 
 @@cjk.k text : Switch to font that supports CJK for *text* @@
 @@cjk.html :: *cjk* @@
 @@cjk.tex  :: { #- @font Noto Sans CJK SC @ *cjk* #- } @@
 @@cjk.txt  :: *cjk* @@
 
-@@fraktur.tex s : \textfrak{*s*} @@
+@@fraktur.k s : Set text *s* in Fraktur @@
+@@fraktur.tex :: \textfrak{*s*} @@
 
-@@euro.html s : &^#x20AC;*s* @@
-@@euro.tex s : €\,*s* @@
+@@euro.k f : The Euro symbol, followed by the amount *f* @@
+@@euro.html :: &^#x20AC;*f* @@
+@@euro.tex,txt :: €\,*f* @@
 
-@@math s :center.bool false : @i *s* @ @@ # xxx
+# TODO: This needs to use MathJax. 
+# @@math s :center.bool false : @i *s* @ @@
 
 @@sub.k base | script : Subscript: @sub x 2 @ @@
-@@sub :: @eval font.Subsuper(K, "sub") eval@ @@
+@@sub.html,tex :: @eval font.Subsuper(K, "sub") eval@ @@
 
 @@sup.k base | script : Superscript: @sup x 2 @ @@
-@@sup :: @eval font.Subsuper(K, "sup") eval@ @@
+@@sup.html,tex :: @eval font.Subsuper(K, "sup") eval@ @@
 
 @@copyright.k : Copyright symbol: © @@
 @@copyright.html :: &copy; @@