pg_basebackup stream xlog to tar

Attached patch adds support for -X stream to work with .tar and .tar.gz
file formats.

If tar mode is specified, a separate pg_xlog.tar (or .tar.gz) file is
created and the data is streamed into it. Regular mode is (should not) see
any changes in how it works.

The implementation creates a "walmethod" for directory and one for tar,
which is basically a set of function pointers that we pass around as part
of the StreamCtl structure. All calls to modify the files are sent through
the current method, using the normal open/read/write calls as it is now for
directories, and the more complicated method for tar and targz.

The tar method doesn't support all things that are required for
pg_receivexlog, but I don't think it makes any real sense to have support
for pg_receivexlog in tar mode. But it does support all the things that
pg_basebackup needs.

Some smaller pieces of functionality like unlinking files on failure and
padding files have been moved into the walmethod because they have to be
differently implemented (we cannot pre-pad a compressed file -- the size
will depend on the compression ration anyway -- for example).

AFAICT we never actually documented that -X stream doesn't work with tar in
the manpage of current versions. Only in the error message. We might want
to fix that in backbranches.

In passing this also fixes an XXX comment about not re-lseeking on the WAL
file all the time -- the walmethod now tracks the current position in the
file in a variable.

Finally, to make this work, the pring_tar_number() function is now exported
from port/tar.c along with the other ones already exported from there.

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