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/bin/pg_basebackup/pg_basebackup.c

@@ -1652,13 +1652,14 @@ BaseBackup(void)
 		maxrate_clause = psprintf("MAX_RATE %u", maxrate);
 
 	basebkp =
-		psprintf("BASE_BACKUP LABEL '%s' %s %s %s %s %s",
+		psprintf("BASE_BACKUP LABEL '%s' %s %s %s %s %s %s",
 				 escaped_label,
 				 showprogress ? "PROGRESS" : "",
 				 includewal && !streamwal ? "WAL" : "",
 				 fastcheckpoint ? "FAST" : "",
 				 includewal ? "NOWAIT" : "",
-				 maxrate_clause ? maxrate_clause : "");
+				 maxrate_clause ? maxrate_clause : "",
+				 format == 't'  ? "TABLESPACE_MAP": "");
 
 	if (PQsendQuery(conn, basebkp) == 0)
 	{