For InnoDB, a truly self referencing FK (a column 'c'  in table
't' referencing column 'c' in table 't') with only one associated
index could be added in an ALTER statement, and made succeeding
index related ALTER operations fail with an assert. Previous MySQL
server versions reject this by requiring supporting and parent
indexes to be to different. This used to be enforced by the SE,
but since the process of choosing supporting and parent indexes
is now done at the SQL layer, the implementation of this check
must also be done at the SQL layer.

Since there are differences in the storage engines support in this
area, a new handlerton foreign key flag is introduced to mark
engines that do not allow the supporting and parent indexes to be
the same.

This patch re-establishes the previous behavior by explicitly
disregarding the supporting index when candidate parent indexes
are considered at the SQL layer, if the storage engine requires
these to be different.

Change-Id: I6f78d5576161b7f3656589efcfde94f2db7dbec0