Create 'default_tablespace' GUC variable that supplies a TABLESPACE

clause implicitly whenever one is not given explicitly.  Remove concept
of a schema having an associated tablespace, and simplify the rules for
selecting a default tablespace for a table or index.  It's now just
(a) explicit TABLESPACE clause; (b) default_tablespace if that's not an
empty string; (c) database's default.  This will allow pg_dump to use
SET commands instead of tablespace clauses to determine object locations
(but I didn't actually make it do so).  All per recent discussions.

src/backend/utils/misc/postgresql.conf.sample

@@ -269,6 +269,7 @@
 # - Statement Behavior -
 
 #search_path = '$user,public'	# schema names
+#default_tablespace = ''	# a tablespace name, or '' for default
 #check_function_bodies = true
 #default_transaction_isolation = 'read committed'
 #default_transaction_read_only = false