Empty search_path in Autovacuum and non-psql/pgbench clients.

This makes the client programs behave as documented regardless of the
connect-time search_path and regardless of user-created objects.  Today,
a malicious user with CREATE permission on a search_path schema can take
control of certain of these clients' queries and invoke arbitrary SQL
functions under the client identity, often a superuser.  This is
exploitable in the default configuration, where all users have CREATE
privilege on schema "public".

This changes behavior of user-defined code stored in the database, like
pg_index.indexprs and pg_extension_config_dump().  If they reach code
bearing unqualified names, "does not exist" or "no schema has been
selected to create in" errors might appear.  Users may fix such errors
by schema-qualifying affected names.  After upgrading, consider watching
server logs for these errors.

The --table arguments of src/bin/scripts clients have been lax; for
example, "vacuumdb -Zt pg_am\;CHECKPOINT" performed a checkpoint.  That
now fails, but for now, "vacuumdb -Zt 'pg_am(amname);CHECKPOINT'" still
performs a checkpoint.

Back-patch to 9.3 (all supported versions).

Reviewed by Tom Lane, though this fix strategy was not his first choice.
Reported by Arseniy Sharoglazov.

Security: CVE-2018-1058

src/bin/scripts/createlang.c

@@ -141,7 +141,7 @@ main(int argc, char *argv[])
 		static const bool translate_columns[] = {false, true};
 
 		conn = connectDatabase(dbname, host, port, username, prompt_password,
-							   progname, false, false);
+							   progname, echo, false, false);
 
 		printfPQExpBuffer(&sql, "SELECT lanname as \"%s\", "
 				"(CASE WHEN lanpltrusted THEN '%s' ELSE '%s' END) as \"%s\" "
@@ -181,7 +181,7 @@ main(int argc, char *argv[])
 			*p += ('a' - 'A');
 
 	conn = connectDatabase(dbname, host, port, username, prompt_password,
-						   progname, false, false);
+						   progname, echo, false, false);
 
 	/*
 	 * Make sure the language isn't already installed