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)

tst/deftype_test.sh

@@ -17,7 +17,7 @@
 # combinations. This test pins down every entry of that table plus the basic
 # behavior of each mode, none of which tst/klammer_test.cpp currently covers.
 #
-# {replace,warn,message} semantics (mac/klammer_set.cpp):
+# {replace,warn,message} semantics (mac/klammer_registry.cpp):
 #   !replace && message  -> Definition_error (nonzero exit)
 #   !replace && empty    -> silently keep the existing definition
 #   replace  && warn     -> emit a warning, then replace