WITH clause in CREATE STATISTICS

Simon just pointed out that having the WITH clause appear in the middle
of the CREATE STATISTICS command looks odd; apparently somebody else
already complained on list about the same. Other commands put the WITH
clause at the end, so perhaps we should do likewise in the new command.

Here's a patch to implement that. I verified that if I change
qualified_name to qualified_name_list, bison does not complain about
conflicts, so this new syntax should support extension to multiple
relations without a problem.

Discuss.

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Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services