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/argument.cpp

@@ -1,8 +1,8 @@
 #include "argument.h"
 #include "argtype.h"
 
-Parameter::Parameter(const std::string& name, Argtype argtype, const Locator& loc,
-                   bool optional, std::string default_value)
+Parameter::Parameter(const std::string& name, const Argtype& argtype, const Locator& loc,
+                   bool optional, const std::string& default_value)
     : m_name(name)
     , m_argtype(argtype)
     , m_optional(optional)