pg_basebackup --xlog compatibility break

In 9.1, the pg_basebackup option --xlog takes no argument.  In 9.2, it
takes a required argument.  I think such compatibility breaks should be
avoided, especially in client-side programs.  Now you can't write a
script running pg_basebackup that works with 9.1 and 9.2, if you need to
include the WAL.

I think the behavior of -x/--xlog should be restored to the state of
9.1, and a new option should be added to select between the fetch and
stream methods.  (With a suitable default, this would also increase
usability a bit.)