Follow practices suggested by "Effective C++"

The patch adjust the code of the base, base-<kernel>, and os repository.
To adapt existing components to fix violations of the best practices
suggested by "Effective C++" as reported by the -Weffc++ compiler
argument. The changes follow the patterns outlined below:

* A class with virtual functions can no longer publicly inherit base
  classed without a vtable. The inherited object may either be moved
  to a member variable, or inherited privately. The latter would be
  used for classes that inherit 'List::Element' or 'Avl_node'. In order
  to enable the 'List' and 'Avl_tree' to access the meta data, the
  'List' must become a friend.

* Instead of adding a virtual destructor to abstract base classes,
  we inherit the new 'Interface' class, which contains a virtual
  destructor. This way, single-line abstract base classes can stay
  as compact as they are now. The 'Interface' utility resides in
  base/include/util/interface.h.

* With the new warnings enabled, all member variables must be explicitly
  initialized. Basic types may be initialized with '='. All other types
  are initialized with braces '{ ... }' or as class initializers. If
  basic types and non-basic types appear in a row, it is nice to only
  use the brace syntax (also for basic types) and align the braces.

* If a class contains pointers as members, it must now also provide a
  copy constructor and assignment operator. In the most cases, one
  would make them private, effectively disallowing the objects to be
  copied. Unfortunately, this warning cannot be fixed be inheriting
  our existing 'Noncopyable' class (the compiler fails to detect that
  the inheriting class cannot be copied and still gives the error).
  For now, we have to manually add declarations for both the copy
  constructor and assignment operator as private class members. Those
  declarations should be prepended with a comment like this:

        /*
         * Noncopyable
         */
        Thread(Thread const &);
        Thread &operator = (Thread const &);

  In the future, we should revisit these places and try to replace
  the pointers with references. In the presence of at least one
  reference member, the compiler would no longer implicitly generate
  a copy constructor. So we could remove the manual declaration.

Issue #465

repos/base/include/util/string.h

@@ -579,42 +579,52 @@ class Genode::String
 		/**
 		 * Number of chars contained in '_buf' including the terminating null
 		 */
-		size_t _len;
+		size_t _len { 0 };
 
 		/**
 		 * Output facility that targets a character buffer
 		 */
-		struct Local_output : Output
+		class Local_output : public Output
 		{
-			char * const _buf;
+			private:
 
-			size_t _num_chars = 0;
+				/*
+				 * Noncopyable
+				 */
+				Local_output(Local_output const &);
+				Local_output &operator = (Local_output const &);
 
-			/**
-			 * Return true if '_buf' can fit at least one additional 'char'.
-			 */
-			bool _capacity_left() const { return CAPACITY - _num_chars - 1; }
+			public:
 
-			void _append(char c) { _buf[_num_chars++] = c; }
+				char * const _buf;
 
-			Local_output(char *buf) : _buf(buf) { }
+				size_t _num_chars = 0;
 
-			size_t num_chars() const { return _num_chars; }
+				/**
+				 * Return true if '_buf' can fit at least one additional 'char'.
+				 */
+				bool _capacity_left() const { return CAPACITY - _num_chars - 1; }
 
-			void out_char(char c) override { if (_capacity_left()) _append(c); }
+				void _append(char c) { _buf[_num_chars++] = c; }
 
-			void out_string(char const *str, size_t n) override
-			{
-				while (n-- > 0 && _capacity_left() && *str)
-					_append(*str++);
-			}
+				Local_output(char *buf) : _buf(buf) { }
+
+				size_t num_chars() const { return _num_chars; }
+
+				void out_char(char c) override { if (_capacity_left()) _append(c); }
+
+				void out_string(char const *str, size_t n) override
+				{
+					while (n-- > 0 && _capacity_left() && *str)
+						_append(*str++);
+				}
 		};
 
 	public:
 
 		constexpr static size_t size() { return CAPACITY; }
 
-		String() : _len(0) { }
+		String() { }
 
 		/**
 		 * Constructor