Implement streaming xlog for backup tools

Add option for parallel streaming of the transaction log while a
base backup is running, to get the logfiles before the server has
removed them.

Also add a tool called pg_receivexlog, which streams the transaction
log into files, creating a log archive without having to wait for
segments to complete, thus decreasing the window of data loss without
having to waste space using archive_timeout. This works best in
combination with archive_command - suggested usage docs etc coming later.

src/bin/pg_basebackup/streamutil.h

@@ -0,0 +1,22 @@
+#include "libpq-fe.h"
+
+extern const char *progname;
+extern char *dbhost;
+extern char *dbuser;
+extern char *dbport;
+extern int	dbgetpassword;
+
+/* Connection kept global so we can disconnect easily */
+extern PGconn *conn;
+
+#define disconnect_and_exit(code)				\
+	{											\
+	if (conn != NULL) PQfinish(conn);			\
+	exit(code);									\
+	}
+
+
+char	   *xstrdup(const char *s);
+void	   *xmalloc0(int size);
+
+PGconn	   *GetConnection(void);