Repair problem identified by Olivier Prenant: ALTER DATABASE SET search_path

should not be too eager to reject paths involving unknown schemas, since
it can't really tell whether the schemas exist in the target database.
(Also, when reading pg_dumpall output, it could be that the schemas
don't exist yet, but eventually will.)  ALTER USER SET has a similar issue.
So, reduce the normal ERROR to a NOTICE when checking search_path values
for these commands.  Supporting this requires changing the API for GUC
assign_hook functions, which causes the patch to touch a lot of places,
but the changes are conceptually trivial.

src/include/access/xlog.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.47 2004/01/06 17:26:23 neilc Exp $
+ * $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.48 2004/01/19 19:04:40 tgl Exp $
  */
 #ifndef XLOG_H
 #define XLOG_H
@@ -222,7 +222,4 @@ extern XLogRecPtr GetRedoRecPtr(void);
  */
 extern XLogRecPtr GetUndoRecPtr(void);
 
-extern const char *assign_xlog_sync_method(const char *method,
-						bool doit, bool interactive);
-
 #endif   /* XLOG_H */