Attempt to re-execute prepared statement or statement in a stored
routine which added foreign key without explicit name resulted in
unwarranted duplicate foreign key name error.

This problem occurred because the code responsible for generation
of names for foreign keys without explicit name stored generated
names in Foreign_key_spec objects (this is necessary for NDB SE
to see these names). During statement re-execution these objects
were reused as is, resulting in attempt to create foreign key with
the same name as during the first execution (since it was assumed
that presence of the name meant that it was provided explicitly).

This fix adds a flag to Foreign_key_spec class to allow to differ
cases when names were provided explicitly from cases when names
were generated. During statement re-execution we re-generate names
if they were not provided explicitly.

Reviewed-by: Sivert Sorumgaard <sivert.sorumgaard@oracle.com>