Extend Tar Format in pg_basebackup

As of now, one can't reliably use tar format to take backup on Windows

because it can't restore tablespaces data which is stored in form of symbolic

links in <data_directory>/pg_tblspc/.  The reason for the same is that  native

windows utilites are not able to create symbolic links while extracting files

from tar.  It might be possible to create symbolic links if cygwin is installed

on your system, however we need this feature to work for native windows as

well.

In PostgreSQL 9.5, a new feature (commit id - 72d422a5) to extend existing

tar format made it possible to reliably take the backup (in tar mode). 

From user perspective, there is nothing much that is changed to take the backup

except that  tar format mode (--format=tar) in pg_basebackup (of the PostgreSQL

9.5 version) will only work with server version 9.5 or later.  This feature is mainly

required for windows, but for the sake consistency it has been changed for all

platforms and also it should enable long (length greater than 99) target symbolic

link for tar format (I think the changes for same are still not done, but we can do

the same now as this feature is committed).

The basic idea behind the feature is that it forms the tablespace map of

all the tablespace symbolic links that are present inside

<data_directory>/pg_tblspc/ and store the same in data_directory for

Exclusive backups (aka backups taken via pg_start_backup() and

pg_stop_backup() functions) and store in backup archive for Non-Exclusive

backups (aka backups taken by pg_basebackup).

The format of tablespace_map file is:

16384 E:\WorkSpace\PostgreSQL\master\tbs

16388 E:\WorkSpace\PostgreSQL\master\tbs              2               3

The tablespace symbolic links are restored during archive recovery and the

tablespace_map file will be renamed to tablespace_map.old at the end of

recovery similar to backup_label file.