Apply patch from Karsten Düsterloh in Bug #28103.

Apply patch from Karsten Düsterloh in Bug #28103.

Karsten writes:

    If the $table_prefix is not empty, several table renamings will happen,
    including for 'pagecontent' and 'mwuser'. This breaks the import into postgres,
    because
    - 'pagecontent' doesn't exist (temporarily renamed to 'text')
    - 'mwuser' won't exist later (temporarily renamed to 'mw_mwuser')
    Also, some sequence names are wrong.
    Furthermore, some of the generated ALTER statements lack the final semicolon

    The attached patch contains the changes I had to do to make this run.