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/xml_node.h

@@ -223,9 +223,9 @@ class Genode::Xml_node
 
 			private:
 
-				Token _token;
-				Token _name;
-				Type  _type;
+				Token _token { };
+				Token _name  { };
+				Type  _type  { INVALID };
 
 			public:
 
@@ -243,7 +243,7 @@ class Genode::Xml_node
 				 * # '/' for marking an empty-element tag (if tag is no end tag)
 				 * # Closing '>' tag delimiter
 				 */
-				Tag(Token start) : _token(start), _type(INVALID)
+				Tag(Token start) : _token(start)
 				{
 					Type supposed_type = START;
 
@@ -294,7 +294,7 @@ class Genode::Xml_node
 				/**
 				 * Default constructor produces invalid Tag
 				 */
-				Tag() : _type(INVALID) { }
+				Tag() { }
 
 				/**
 				 * Return type of tag
@@ -343,8 +343,8 @@ class Genode::Xml_node
 		{
 			private:
 
-				Token _next;   /* token following the comment */
-				bool  _valid;  /* true if comment is well formed */
+				Token _next  { };        /* token following the comment */
+				bool  _valid { false };  /* true if comment is well formed */
 
 			public:
 
@@ -353,7 +353,7 @@ class Genode::Xml_node
 				 *
 				 * \param start  first token of the comment tag
 				 */
-				Comment(Token t) : _valid(false)
+				Comment(Token t)
 				{
 					/* check for comment start */
 					if (!t.matches("<!--"))
@@ -370,7 +370,7 @@ class Genode::Xml_node
 				/**
 				 * Default constructor produces invalid Comment
 				 */
-				Comment() : _valid(false) { }
+				Comment() { }
 
 				/**
 				 * Return true if comment is valid
@@ -748,14 +748,6 @@ class Genode::Xml_node
 			catch (Nonexistent_sub_node) { return true; }
 		}
 
-		/**
-		 * Return true if node is the last of a node sequence
-		 *
-		 * \noapi
-		 * \deprecated use 'last' instead
-		 */
-		bool is_last(const char *type = 0) const { return last(); }
-
 		/**
 		 * Return sub node with specified index
 		 *