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)

sks/kutil/klammer_base.h

@@ -18,14 +18,14 @@ public:
         {};
     virtual ~Klammer_base() {};
 
-    std::string get(std::string);
+    std::string get(const std::string& name);
     std::string get(const char* name);
     virtual std::string html();
     virtual std::string tex();
     virtual std::string txt();
 
     std::string result();
-    void show(std::string klammer_name);
+    void show(const std::string& klammer_name);
 
     Machine m_machine;
 };