Improve initdb's query for generating default descriptions a little.

While poking into initdb's performance, I noticed that this query
wasn't being done very intelligently.  By forcing it to execute
obj_description() for each pg_proc/pg_operator join row, we were
essentially setting up a nestloop join to pg_description, which
is not a bright query plan when there are hundreds of outer rows.
Convert the check for a "deprecated" operator into a NOT EXISTS
so that it can be done as a hashed antijoin.  On my workstation
this reduces the time for this query from ~ 35ms to ~ 10ms.
Which is not a huge win, but it adds up over buildfarm runs.

In passing, insert forced query breaks (\n\n, in single-user mode)
after each SQL-query file that initdb sources, and after some
relatively new queries in setup_privileges().  This doesn't make
a lot of difference normally, but it will result in briefer, saner
error messages if anything goes wrong.

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