In a trigger or a function used in a statement it is possible to do
SELECT from a table being modified by the statement.  However,
encapsulation of such SELECT into a view and selecting from a view
instead of direct SELECT was not possible.

This happened because tables used by views (which in their turn
were used from functions/triggers) were not excluded from checks
in unique_table() routine as it happens for the rest of tables
added to the statement table list for prelocking.

With this fix we ignore all such tables in unique_table(), thus
providing consistency: inside a trigger or a functions SELECT from
a view may be used where plain SELECT is allowed.  Modification of
the same table from function or trigger is still disallowed.  Also,
this patch doesn't affect the case where SELECT from the table being
modified is done outside of function of trigger, such SELECTs are
still disallowed (this limitation and visibility problem when function
select from a table being modified are subjects of bug 21326).  See
also bug 22427.