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)

mac/argv.h

@@ -58,7 +58,13 @@ public:
     // space-joined; given(name) distinguishes "--name with no words"
     // from an absent --name.  Used for subcommand-style interfaces
     // (kdesc --font install <dir>).
-    void var(const std::string& name, const std::string& desc);
+    // A variadic option: the words after the flag, up to the next one.
+    // `parameter` is shown in the usage line as "[<parameter>]" -- an option
+    // whose words are a free argument (a search phrase) should name it, so
+    // the usage says what may follow; a subcommand-style option (--font,
+    // --coverage) leaves it empty and documents its words in its own help.
+    void var(const std::string& name, const std::string& desc,
+             const std::string& parameter = "");
 
     void update_width(Arg arg);