There is a need in MDEV-25292 to have both C_ALTER_TABLE and
select_field_count in one call. Semantically creation mode and field
count are two different things. Making creation mode negative
constants and field count positive variable into one parameter seems
to be a lazy hack for not making the second parameter.

select_count does not make sense without alter_info->create_list, so
the natural way is to hold it in Alter_info too. select_count is now
stored in member select_field_count.