Move WAL warning

When running pg_basebackup with -x to include all transaction log, the
server will still throw a warning about xlog archiving if it's not
enabled - that is completely irrelevant since pg_basebackup has
included it already (and if it was gone, the base backup step itself
will fail - actual error and not warning).

This patch moves the warning from do_pg_base_backup to pg_base_backup,
so it still shows when using the explicit function calls, but goes
away when using pg_basebackup.

Objections?

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/