* varobj.h (varobj_set_frozen): New

    * varobj.h (varobj_set_frozen): New
        (varobj_get_frozen): New.
        (varobj_update): New parameter explicit.
        * varobj.c (struct varobj): New fields frozen
        and not_fetched.
        (varobj_set_frozen, varobj_get_frozen): New.
        (install_new_value): Don't fetch values for
        frozen variable object, or children thereof.  Allow
        a frozen variable object to have non-fetched value.
        (varobj_update): Allow updating child variables.
        Don't traverse frozen children.
        (new_variable): Initialize the frozen field.
        (c_value_of_variable): Return NULL for frozen
        variable without any value yet.
        * mi/mi-cmd-var.c (varobj_update_one): New parameter
        'explicit'.
        (mi_cmd_var_create): Output the 'frozen' field,
        as soon as testsuite is adjusted to expect that field.
        (mi_cmd_var_set_frozen): New.
        (mi_cmd_var_update): Pass the 'explicit' parameter to
        varobj_update_one.
        * mi/mi-cmds.c (mi_cmds): Register '-var-set-frozen'.
        * mi/mi-cmds.h (mi_cmd_var_set_frozen): Declare.