extract our code that fetches table names

keep function private, so far we use it in upgrade
scripts and tests.
require newer DBD::Pg on Pg 9.2 for upgrade

We use DBI's table_info method in upgrade scripts.
It fails on Pg 9.2 with old DBD::Pg driver. We don't
have to bump required version for all Pg users, but
can help users of Pg 9.2 to solve the problem by
providing better error message.
Document the new DBD::Pg requirement

Hopefully folks will install the newer DBD::Pg before upgrading, but if
they don't, document how to re-start since the presence of a schema file
makes it impossible to just restart the make upgrade-database.
Merge branch '4.0/pg-9.2-compatibility' into 4.0-trunk

Conflicts:
	docs/UPGRADING-4.0