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)

tst/coverage/basic.k

@@ -0,0 +1,29 @@
+# Fixture for coverage_test.sh.  Engine tier: it declares its own targets,
+# so the suite runs with no klammer set.
+@@@target ta | Target A @@@
+@@@target tb | Target B @@@
+@@@target tc | Target C @@@
+
+# Defined per target: coverage is what the definitions say.
+@@ab.k s : defined for ta and tb @@
+@@ab.ta :: A*s* @@
+@@ab.tb :: B*s* @@
+
+# A comma list is the same statement written once (step 1).
+@@cd.k s : defined for ta and tc by a list @@
+@@cd.ta,tc :: C*s* @@
+
+# General body of plain text: every target.
+@@plain : just text @@
+
+# General body calling one klammer: that klammer's coverage.
+@@calls_ab s : @ab *s* @ @@
+
+# Intersection of two klammers with different coverage: only ta is in both.
+@@calls_both s : @ab *s* @ @cd *s* @ @@
+
+# ".*" is an assertion: every target, including targets not yet defined.
+# Distinct from a bare definition, which asserts nothing -- that is the
+# writer's macro form.
+@@universal.k s : works for any target @@
+@@universal.* :: U*s* @@