Non-colliding auto generated names

This patch makes all forms of autogenerated unique, primary key and sequence
names avoid collisions. (Well, as much as you can do without "locking" your
chosen name).

This addresses a long standing annoyance I experience whenever I rename a
table to table_old and then try to create a table with the same name. It
also addresses a concern raised at linux.conf.au.

I suggest that someone inspect the patch to determine if my "overloading" of
CreateIndexName is appropriate (for sequences, etc.)

All regression tests pass. The attached SQL script will run fine under the
patch, whereas before it would have failed miserably.

Chris