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)

com/kdiag.cpp

@@ -45,7 +45,7 @@ int main(int argc, char* argv[])
             args.usage(file_basename(argv[0]));
             exit(1);
         }
-        auto p = [&](std::string name) { return args.get(name) == "true"; };
+        auto p = [&](const std::string& name) { return args.get(name) == "true"; };
 
         args.parse(argc, argv);
         verbose_level = args.as_int("v");