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/util.cpp

@@ -134,6 +134,36 @@ strings_t word_split(const std::string& s)
     return regex_split(s, std::regex("\\s+"));
 }
 
+std::string collapse_whitespace(const std::string& s)
+{
+    std::string result {};
+    bool in_space = true;                 // leading whitespace is dropped
+    for (char c : s) {
+        if (std::isspace(static_cast<unsigned char>(c)) != 0) {
+            in_space = true;
+        } else {
+            if (in_space && !result.empty()) {
+                result += ' ';
+            }
+            in_space = false;
+            result += c;
+        }
+    }
+    return result;
+}
+
+bool contains_fold(const std::string& haystack, const std::string& needle)
+{
+    auto fold = [](const std::string& s) {
+        std::string result {};
+        for (char c : s) {
+            result += static_cast<char>(std::tolower(static_cast<unsigned char>(c)));
+        }
+        return result;
+    };
+    return fold(haystack).find(fold(needle)) != std::string::npos;
+}
+
 bool is_in(const std::string& s, const strings_t& v)
 {
     return find(v.begin(), v.end(), s) != v.end();