Drop index query on an ndb table crashes the server if the table has
foreign keys. Internally, 'drop index' query is mapped to an 'alter table'
query. But the thd->lex->sql_command value is different for the queries.
For alter table, the value is SQLCOM_ALTER_TABLE and for drop index it
is SQLCOM_DROP_INDEX. and the functions create and drop_table_impl
don't handle SQLCOM_DROP_INDEX query type. This is fixed by proper
handling of SQLCOM_DROP_INDEX type in the above said functions.

This patch also fixes the following issue.

For a table without any primary key, mysqld server passes any available,
not nullable column with an unique key as the implicit primary key. If
an 'alter table' either dropping the unique index on that column or
removing the constraint not null is issued, the implicit primary key
gets changed. If any foreign key was referencing that column, during the
alter query, its parent key has to be updated. But
copy_fk_for_offline_alter() function while copying the foreign keys from
the old table to the new table, copies the parent index as it is, if the
index was previously a primary key. This leads to an inconsistent
foreign key constraint.

This patch fixes that by checking if the pk is intact during the alter.
It either preserves the fk's parent key if the pk is intact or else
finds for a better matching index. If a matching index is not found, it
returns an error and the alter is aborted.