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/show.h

@@ -8,9 +8,10 @@
 #include "argtype.h"
 #include "argument.h"
 #include "klammer.h"
-#include "klammer_set.h"
+#include "klammer_registry.h"
+#include "klammerset_registry.h"
 #include "state.h"
-#include "target_set.h"
+#include "target_registry.h"
 #include "machine.h"
 #include "file.h"
 
@@ -143,16 +144,22 @@ std::ostream& operator<<(std::ostream& os, const Klammer::variable_map_t& vm);
 // Klammer::components
 std::ostream& operator<<(std::ostream& os, const Klammer::components& kc);
 
-// Klammer_set
-std::ostream& operator<<(std::ostream& os, const Klammer_set& ks);
+// Klammer_registry
+std::ostream& operator<<(std::ostream& os, const Klammer_registry& ks);
 
 // Target
 std::ostream& operator<<(std::ostream& os, const Target& t);
 
-// Target_set
-std::ostream& operator<<(std::ostream& os, const Target_set& ts);
+// Target_registry
+std::ostream& operator<<(std::ostream& os, const Target_registry& ts);
+
+// Klammerset
+std::ostream& operator<<(std::ostream& os, const Klammerset& ks);
+
+// Klammerset_registry
+std::ostream& operator<<(std::ostream& os, const Klammerset_registry& kr);
 
 // Machine
 std::ostream& operator<<(std::ostream& os, const Machine& m);
 
-void modify_stream(std::string name);
+void modify_stream(const std::string& name);