The problem happened because Item_ident_for_show::field_type() always
returned MYSQL_TYPE_DOUBLE and ignored the actual data type of the
referenced Field. As a result, the execution always used
Item_ident_for_show::val_real() to send the default value of the field,
so most default values for non-numeric types were displayed as '0'.

This patch:
1. Cleanup:
   a. Removes Send_field::charsetnr, as it's been unused since
      introduction of Item::charset_for_protocol() in MySQL-5.5.
   b. Adds the "const" qualifier to Field::char_length().
      This is needed for (5.a), see below.

2. Introduces a new virtual method Type_handler::charset_for_protocol(),
   returning item->collation.collation for string data types, or
   &my_charset_bin for non-string data types.

3. Changes Item::charset_for_protocol() from virtual to non-virtual.
   It now calls type_handler()->charset_for_protocol().
   As a good side effect, duplicate code in Item::charset_for_protocol() and
   Item_temporal_hybrid_func::charset_for_protocol() is now gone.

4. Fixes Item_ident_for_show::field_type() to correctly return
   its data type according to the data type of the referenced field.
   This actually fixes the problem reported in MDEV-11672.
   Now the default value is sent using a correct method, e.g.
   val_str() for VARCHAR/TEXT, or val_int() for INT/BIGINT.
   This required additional changes:
   a. in DBUG_ASSERT in Protocol::store(const char *,size_t,CHARSET_INFO),
      This method is now used by mysqld_list_fields(), which
      (unlike normal SELECT queries) does not set
      field_types/field_pos/field_count.
   b. Item_ident_for_show::Item_ident_for_show() now set standard attributes
      (collation, decimals, max_length, unsigned_flag) according to the
      referenced field, to make charset_for_protocol() return the correct
      value and to make mysqld_list_fields() correctly send default
      values.

5. In order to share the code between Item_field::set_field() and
   Item_ident_for_show::Item_ident_for_show():
   a. Introduces a new method Type_std_attributes::set(const Field*)
   b. To make (a) possible, moves Item::fix_char_length() from Item
      to Type_std_attributes, also moves char_to_byte_length_safe()
      from item.h to sql_type.h
   c. Additionally, moves Item::fix_length_and_charset() and
      Item::max_char_length() from Item to Type_std_attributes.
      This is not directly needed for the fix and is done just for symmetry
      with fix_char_length(), as these three methods are directly related
      to each other.