Add option to include WAL in base backup

When included, this makes the base backup a complete working
"clone" of the initial database, ready to have a postmaster
started against it without the need to set up any log archiving
or similar.

Magnus Hagander, reviewed by Fujii Masao and Heikki Linnakangas

src/backend/replication/repl_scanner.l

@@ -61,6 +61,7 @@ FAST			{ return K_FAST; }
 IDENTIFY_SYSTEM		{ return K_IDENTIFY_SYSTEM; }
 LABEL			{ return K_LABEL; }
 PROGRESS			{ return K_PROGRESS; }
+WAL			{ return K_WAL; }
 START_REPLICATION	{ return K_START_REPLICATION; }
 ","				{ return ','; }
 ";"				{ return ';'; }