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/katom.cpp

@@ -281,7 +281,7 @@ katom_list trim(const katom_list& katoms, bool trim_inactive)
     return trim(result, trim_types);
 }
 
-strings_t line_split(std::string s)
+strings_t line_split(const std::string& s)
 {
     std::istringstream is(s);
     strings_t result {};
@@ -293,7 +293,7 @@ strings_t line_split(std::string s)
     return result;
 }
 
-std::pair<std::string, strings_t> line_split(fs::path pathname)
+std::pair<std::string, strings_t> line_split(const fs::path& pathname)
 {
     std::string source {};
     strings_t lines {};