configuration: support multiple configuration files

src/utility/option_parser.h

@@ -106,8 +106,18 @@ struct option_parser
 			m_defined = true;
 		}
 
-		bool defined() const { return m_defined; }
-		void run_default() const { m_default(); }
+		bool defined() const
+		{
+			return m_defined;
+		}
+
+		void run_default()
+		{
+			if (m_defined)
+				throw std::runtime_error("option already defined");
+			m_default();
+			m_defined = true;
+		}
 
 	private:
 		bool m_defined;

src/utility/string.h

@@ -32,8 +32,8 @@ template <size_t N> size_t const_strlen(const char (&)[N]) {
 }
 
 // it's present in boost for std::string, but we want more general version.
-template <typename CollectionT, typename StringT>
-StringT join(CollectionT &&collection, StringT &&separator)
+template <typename StringT, typename CollectionT>
+StringT join(const CollectionT &collection, const StringT &separator)
 {
 	StringT result;
 	auto first = std::begin(collection), last = std::end(collection);