Add noexcept to conform with C++11

[viboes]

I'm adding BOOST_NOEXCEPT specification to Boost.Thread (#7279), and I can not complete it as Boost.System lacks this feature.

    class error_category {
    public:
      virtual const char* name() const noexcept = 0;
      virtual error_condition default_error_condition(int ev) const noexcept;
      virtual bool equivalent(int code, const error_condition& condition) const noexcept;
      virtual bool equivalent(const error_code& code, int condition) const noexcept;
      bool operator==(const error_category& rhs) const noexcept;
      bool operator!=(const error_category& rhs) const noexcept;
      bool operator<(const error_category& rhs) const noexcept;
    };
    const error_category& generic_category() noexcept;
    const error_category& system_category() noexcept;


    class error_code {
    public:
        error_code() noexcept;
        error_code(int val, const error_category& cat) noexcept; 
        template <class ErrorCodeEnum>
        error_code(ErrorCodeEnum e) noexcept;
        void assign(int val, const error_category& cat) noexcept; 
        template <class ErrorCodeEnum>
        errorcode& operator=(ErrorCodeEnum e) noexcept;
        void clear() noexcept;
        int value() const noexcept;
        const error_category& category() const noexcept; 
        error_condition default_error_condition() const noexcept; 
        explicit operator bool() const noexcept;
    };
    error_code make_error_code(errc e) noexcept;
    bool operator<(const error_code& lhs, const error_code& rhs) noexcept;

    class error_condition {
    public:
        error_condition() noexcept;
        error_condition(int val, const error_category& cat) noexcept; 
        template <class ErrorConditionEnum>
        error_condition(ErrorConditionEnum e) noexcept;
        void assign(int val, const error_category& cat) noexcept;         
        template<class ErrorConditionEnum>
        error_condition& operator=(ErrorConditionEnum e) noexcept;
        void clear() noexcept;
        int value() const noexcept;
        const error_category& category() const noexcept; 
        explicit operator bool() const noexcept;
    };
    bool operator<(const error_condition& lhs, const error_condition& rhs) noexcept;

Any plans to take care of this?