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/document/document.k

@@ -62,60 +62,4 @@
 : Top-level document structure 
 @@
 
-@@document :: @eval :cpp *KLAMMERTEXT_HOME*/sks/document/document document @ @@
-
-# @link https://cdn.britannica.com/09/152309-050-5E0B2A42/Sahara-Morocco.jpg :text Niagara @
-
-@@niagara :
-@link https://cdn.britannica.com/09/152309-050-5E0B2A42/Sahara-Morocco.jpg :text Niagra @
-@@
-
-#[
-@@niagara.html :
-<a href="https://cdn.britannica.com/09/152309-050-5E0B2A42/Sahara-Morocco.jpg">Niagara</a>
-@@
-
-@@niagara.tex :
-@link https://cdn.britannica.com/09/152309-050-5E0B2A42/Sahara-Morocco.jpg :text Niagra @
-@@
-]#
-
-
-# @@moby : Moby @@
-@@moby :
-But here is an artist.  He desires to paint you the dreamiest,
-shadiest, quietest, most enchanting bit of romantic landscape in all
-the valley of the Saco.  What is the chief element he employs?  There
-stand his trees, each with a hollow trunk, as if a hermit and a
-crucifix were within; and here sleeps his meadow, and there sleep his
-cattle; and up from yonder cottage goes a sleepy smoke. Deep into
-distant woodlands winds a mazy way, reaching to overlapping spurs of
-mountains bathed in their hill-side blue.  But though the picture lies
-thus tranced, and though this pine-tree shakes down its sighs like
-leaves upon this shepherd's head, yet all were vain, unless the
-shepherd's eye were fixed upon the magic stream before him.  Go visit
-the Prairies in June, when for scores on scores of miles you wade
-knee-deep among Tiger-lilies---what is the one charm wanting? Water
----there is not a drop of water there! Were @niagara@ but a cataract of
-sand, would you travel your thousand miles to see it?
-@@
-
-@@smoby.html :
-Go visit the Prairies in June, when for scores on scores of miles you
-wade knee-deep among Tiger-lilies---what is the one charm wanting?
-Water --- there is not a drop of water there! Were @niagara@ but a
-cataract of sand, would you travel your thousand miles to see it?
-@@
-
-@@smoby.tex :
-Go visit the Prairies in June, when for scores on scores of miles you
-wade knee-deep among Tiger-lilies---what is the one charm wanting?
-Water --- there is not a drop of water there! Were @niagara@ but a
-cataract of sand, would you travel your thousand miles to see it?
-@@
-
-@@ssmoby :
-Go visit the Prairies in June, when for scores on scores of miles you
-wade knee-deep among Tiger-lilies---what is the one charm wanting?
-Water---there is not a drop of water there!
-@@
+@@document.html,tex :: @eval :cpp *KLAMMERTEXT_HOME*/sks/document/document document @ @@