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/code/code.k

@@ -6,15 +6,16 @@
 A source file displayed verbatim
 @@
 
-@@code :: @eval code_block.Code(K) @ @@
+@@code.html,tex :: @eval code_block.Code(K) @ @@
 
 @@c.k code_text :
 A word or phrase displayed verbatim in a line
 @@
 
-@@c :: @eval code_block.Code_fragment(K) eval@
+@@c.html,tex :: @eval code_block.Code_fragment(K) eval@
 @@
 
 # :cwd makes the filename resolve against the DOCUMENT's directory, not
 # the directory ktext happens to run in.
-@@source_file filename : @eval :cwd *K_input_dir* code_block.Source(K) @ @@
+@@source_file.k filename : Display the contents of the file verbatim. @@
+@@source_file.html,tex :: @eval :cwd *K_input_dir* code_block.Source(K) @ @@