Define cstring_tie::end()

std::ranges::range<T> requires that T provide a reasonable `end()`.
cstring_tie did not provide one since it uses private inheritance of
`std::array`.  However, it can provide a reasonable one, so define that
here.

common/include/varutil.h

@@ -25,6 +25,10 @@ public:
 	unsigned count() const { return m_count; }
 	const char *string(std::size_t i) const { return this->operator[](i); }
 	using array_t::begin;
+	auto end() const
+	{
+		return std::next(begin(), m_count);
+	}
 private:
 	unsigned m_count;
 };