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/target/target.k

@@ -3,7 +3,7 @@ The targets in the SKS use the LaTeX convention for converting ASCII
 characters into standard typographical characters.  This is done
 by LaTeX by default; other targets must use the transforms parameter
 of Parameter_set, as defined for the m_parameters variable of the
-Target_set class.  (See file target_set.cpp.)
+Target_registry class.  (See file target_registry.cpp.)
 
 The LaTeX transformations supported by the SKS are: