IN ALTER TABLE

The problem was that if in-place ALTER TABLE was killed while
executing handler functions, the ALTER TABLE would end with
ER_QUERY_INTERRUPTED but the ALTER TABLE operations
could still have completed and thus changed the table. This
also meant that the statement would not be binlogged thus
breaking replication.

The reason was that ALTER TABLE would check if it was killed
after the changes had been committed, and report statement
failure in this case even if the table had already been updated.

This patch fixes the problem by not checking if the statement
has been killed after the main phase of ALTER TABLE has completed.
This means that once the table changes have been made, killing
ALTER TABLE will not be possible. This is implemented by adding
a new MYSQL_OPEN_IGNORE_KILLED flag that is set when
reopening tables and that open_table() and mysql_lock_tables()
will obey.