with more than 16 items

Regression from adding COLUMNS partitioning.

When parsing the ALTER TABLE t ADD PARTITION (PARTITION p VALUES IN (...))
we first allocate a buffer for 16 values, which is the maximum number of columns in
COLUMNS partitioning.
If more than 16 values, we conclude that it must be partitioned by a single field
and it is really a list of single field values and convert it to this in
partition_info::reorganize_into_single_field_col_val and set the num_columns to 1
But after this there was no allocation for further elements in add_column_value.

Fixed by
1) allow reorganize_into_single_field_col_val to reorganize even if the number of
columns is unknown (assuming that it must be single field).
2) Before calling add_column_value() after reorganize_into_single_field_col_val()
is done, call init_column_part() to allocate a new list element.

Also added a few comments to member variables.
Also changed return type from int to bool for a couple of functions.