Map basebackup tablespaces using a tablespace_map file

Windows can't reliably restore symbolic links from a tar format, so
instead during backup start we create a tablespace_map file, which is
used by the restoring postgres to create the correct links in pg_tblspc.
The backup protocol also now has an option to request this file to be
included in the backup stream, and this is used by pg_basebackup when
operating in tar mode.

This is done on all platforms, not just Windows.

This means that pg_basebackup will not not work in tar mode against 9.4
and older servers, as this protocol option isn't implemented there.

Amit Kapila, reviewed by Dilip Kumar, with a little editing from me.

src/include/access/xlog.h

@@ -17,6 +17,8 @@
 #include "access/xlogreader.h"
 #include "datatype/timestamp.h"
 #include "lib/stringinfo.h"
+#include "nodes/pg_list.h"
+#include "storage/fd.h"
 
 
 /* Sync methods */
@@ -258,7 +260,9 @@ extern void assign_checkpoint_completion_target(double newval, void *extra);
  * Starting/stopping a base backup
  */
 extern XLogRecPtr do_pg_start_backup(const char *backupidstr, bool fast,
-				   TimeLineID *starttli_p, char **labelfile);
+				   TimeLineID *starttli_p, char **labelfile, DIR *tblspcdir,
+				   List **tablespaces, char **tblspcmapfile, bool infotbssize,
+				   bool needtblspcmapfile);
 extern XLogRecPtr do_pg_stop_backup(char *labelfile, bool waitforarchive,
 				  TimeLineID *stoptli_p);
 extern void do_pg_abort_backup(void);
@@ -267,4 +271,7 @@ extern void do_pg_abort_backup(void);
 #define BACKUP_LABEL_FILE		"backup_label"
 #define BACKUP_LABEL_OLD		"backup_label.old"
 
+#define TABLESPACE_MAP			"tablespace_map"
+#define TABLESPACE_MAP_OLD		"tablespace_map.old"
+
 #endif   /* XLOG_H */