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/link/link.k

@@ -1,79 +1,23 @@
-@@link target
+@@link.k target
 :text
 :footnote.bool false 
 :section.bool false
 :nq.bool false
-:basename # xxx
+:basename # xxx -- Deprecated?
 :code # xxx
 :phrase # xxx
 :dox # xxx
 :doxanchor # xxx     
 :id # xxx
-:
-  @eval link.Link(K) eval@
+: Links within documents and to external URLs.
 @@
 
-@@target id :text :
-  @eval link.Target(K) eval@
-@@
+@@link.html,tex,txt :: @eval link.Link(K) eval@ @@
 
-@@email name | domain :subject :body :
-  @eval link.Email(K) eval@          
-@@
+@@target.k id :text : A link target within a document @@
 
-#[
-@@@category link
-target link
-:desc Links @@@
+@@target.html,tex :: @eval link.Target(K) eval@ @@
 
-@@target.doc id :text :
-Link target in page is @i-name, with optional @i-text for target @@
+@@email.k name | domain :subject :body : An email with an mangled address for HTML @@
 
-@@target : @eval link.target(K) eval@ @@
-
-@@link.doc url
-:text 
-:download.bool false 
-:mail.bool false 
-:nopage.bool false
-:section.bool false
-:pageonly.bool false 
-:footnote.bool true 
-:in_footnote.bool false
-:dox.bool false
-:abs.bool false
-:add_number.bool false
-:
-Link to @i-url using @i-text for display, or @i-url if @i-text is not
-specified.  If :section is true, the @i-url is the text of a section
-title. 
-@@
-
-@@link : @eval link.link(K) eval@ @@
-
-@@subpage kt_basename
-:pagetitle
-:sectiontitle
-:linktext
-:index
-:toc.bool true
-:logo
-:include
-:prevlink
-:nextlink
-:uplink 
-:logo
-:top_level.int
-:href_base
-:search.bool false
-:
-@eval link.subpage(K) eval@ @@
-
-@@subpages.html s :
-<!--toc-->
-*s*
-<!--toc-->
-@@
-
-@@subpages.latex s : *s* @@
-]#
+@@email.html,tex :: @eval link.Email(K) eval@ @@