base: Add const qualifiers to 'util/list.h'

This patch adds support for iterating through a const list. This allows
users of lists to be more rigid with regard to constness. Furthermore,
the patch adds the function 'List::insert_at' for inserting an element
at a specified position. By adding this function, we can remove code
duplication in nitpicker.

gems/src/server/d3m/input_service.h

@@ -157,7 +157,7 @@ namespace Input {
 			 *
 			 * \return  total number of available input events
 			 */
-			Genode::size_t flush_sources(Event *dst, Genode::size_t dst_max) const
+			Genode::size_t flush_sources(Event *dst, Genode::size_t dst_max)
 			{
 				Genode::size_t dst_count = 0;