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/replication/basebackup.h

@@ -21,6 +21,16 @@
 #define MAX_RATE_UPPER	1048576
 
 
+typedef struct
+{
+	char	   *oid;
+	char	   *path;
+	char	   *rpath;			/* relative path within PGDATA, or NULL */
+	int64		size;
+} tablespaceinfo;
+
 extern void SendBaseBackup(BaseBackupCmd *cmd);
 
+extern int64 sendTablespace(char *path, bool sizeonly);
+
 #endif   /* _BASEBACKUP_H */