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/interfaces/ecpg/pgtypeslib/dt.h

@@ -298,7 +298,6 @@ int			EncodeInterval(struct tm * tm, fsec_t fsec, int style, char *str);
 int			tm2timestamp(struct tm *, fsec_t, int *, timestamp *);
 
 int			DecodeUnits(int field, char *lowtoken, int *val);
-bool		ClearDateCache(bool, bool, bool);
 
 bool		CheckDateTokenTables(void);