Klammerset: the @@@klammerset construct, its search path, and const correctness

The @@@klammerset system command formally declares a klammerset — a
named, logically related group of klammer definitions — with an
operative, idempotent declaration (:requires and :files load in order
at the declaration point, relative to the declaring file). A bare
symbol given to ktext -k, kdesc --input, or :requires resolves to
x/x.k on the search path: the document's directory, then
KLAMMERTEXT_KLAMMERSETS, then KLAMMERTEXT_HOME; kdesc --klammerset
lists the available sets. sks/sks.k is the first declared klammerset,
so `-k sks` loads the SKS by name. The engine's lookup classes were
renamed *_set → *_registry to keep the two concepts apart, and the
whole C++ tree now follows standard const-correctness conventions.
tst/ gains klammerset_test.sh (18 cases).

(from dev 64b1abf23e56)

mac/argtype.h

@@ -25,9 +25,10 @@ public:
         , m_loc()
         {};
 
-    Argtype(std::string name, std::string desc, std::string symbolic_pattern, std::string pattern,
-            std::string default_value, std::string alone_value,
-            std::string python_cast, modify_string_f python_format,
+    Argtype(const std::string& name, const std::string& desc, const std::string& symbolic_pattern,
+            const std::string& pattern,
+            const std::string& default_value, const std::string& alone_value,
+            const std::string& python_cast, const modify_string_f& python_format,
             const Locator& loc);
 
     std::string python_value(const std::string& var_name, std::vector<std::string> value, size_t name_size);