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/argv.cpp

@@ -227,7 +227,7 @@ void Argv::usage(const std::string& command)
 }
 
 
-void Argv::check_flags_and_options(std::string command, strings_t& words)
+void Argv::check_flags_and_options(const std::string& command, strings_t& words)
 {
     std::vector<std::string> not_defined {};
     for (auto word : words) {
@@ -334,10 +334,10 @@ void Argv::parse_optional(strings_t& words, string_map& named_args)
     // std::cout << "\n";
 }
 
-void Argv::parse_positional(std::string command, //strings_t words,
+void Argv::parse_positional(const std::string& command, //strings_t words,
                             std::string pos_args, string_map& named_args)
 {
-    for (std::string req : m_req_names) {
+    for (const std::string& req : m_req_names) {
         auto arg = m_args[req];
         auto [substring, rest, found] = regex_split_prefix(arg.m_rgx, pos_args);
         if (!found) {