Follow-up to r15791: Rename "user" and "text" when upgrading on PostgreSQL

Follow-up to r15791: Rename "user" and "text" when upgrading on PostgreSQL

You can lie to me, but not to your installer.

Make DatabasePostgres::tableExists to check
for real table names, not faked ones.

DatabasePostgres is currently lying to the rest
of the MediaWiki that "mwuser" table is actually
called "user" and that "pagecontents" is called
"text". While MediaWiki does not care, the
installer (and updater do).

This allows us to overcome first hurdle
in getting MediaWiki 1.7.3 to update to trunk
on PostgreSQL and uncover further bugs.

For this commit to actually do something,
we rename those tables when upgrading to match
what we have in maintenance/postgres/tables.sql

And by the way, tell installer not to check
for "user" table, since most PostgreSQL users
will have "mwuser" instead. Picking "archive"
instead.