PostgreSQL: Port update-keys.sql to PostgreSQL

This fixes the same bug in PostgreSQL that was reported
against Oracle as bug 71040, using the same method of copying
the update-keys.sql script into maintenance/postgres.

Since all three copies of this file do the same thing, perhaps
we should find lowest-common-denominator syntax that works in
all databases to avoid redundant copies that can get out of
sync with each other.  (The Oracle and PostgreSQL versions are
already identical to each other).

The comments in the file are confusing and ungrammatical, but
they are a copy of the same language from the other copies.
Since I don't know what it is trying to say, I can't
fix it.

I have verified that this patch fixes the problem where
mediawiki could not be installed with PostgreSQL using
either the CLI or the web installer, due to SQL syntax errors.

I haven't tested the the update-keys actually accomplishes
whatever it was introduced to accomplish, though.

Bug: 72834
Change-Id: I2a0cfa3dd0751b9fb65450b1537b6e77be60009a