feat(argtype): :alone - the value of an option written without one

An optional argument has three values: the default (the name is absent),
the argument type's :alone value (the name is written alone), and a
written value.

:alone is declared by the argument type only, never by a klammer's
parameter declaration -- a default is what one klammer means by silence,
but a bare option name must read the same way in every klammer.  The bool
type declares :alone true, which is the whole of the convention that a
bare boolean option means true; there is no boolean special case in the
engine.  A type whose pattern matches running text cannot declare :alone,
since an option's value runs to the next bar or option name and would
swallow the following text.

kdesc and `ktext -m` now show [default: X] and [alone: Y] per argument
type.

In the Standard Klammer Set: @code :number becomes a bool (it was an
untyped string tested only for truthiness, so a bare :number was a
no-op); decimal_mark declares :alone comma; table_hline and table_vline
declare :alone all.  The 35 bools that default false gained the bare form
for free.

Tests: tst/alone_test.sh (21 cases) joins the shipped suite.

(from dev 6024f49c2859)

mac/argtype_set.h

@@ -19,6 +19,7 @@ public:
     void check_for_existing_definition(const std::string& name, const Locator& loc);
     void add(const std::string& name, const std::string& desc,
              const std::string& pattern, const std::string& default_value,
+             const std::string& alone_value,
              const std::string& python_cast, modify_string_f python_format,
              const Locator& loc);
     void add(std::vector<Katom>::iterator begin, std::vector<Katom>::iterator end, std::vector<Katom>& katoms);