Manual anti-wraparound vacuums

In another thread, Alvaro quoted from the manual:
> If for some reason autovacuum fails to clear old XIDs from a table, the
> system will begin to emit warning messages like this when the database's oldest
> XIDs reach ten million transactions from the wraparound point:

> WARNING: database "mydb" must be vacuumed within 177009986 transactions
> HINT: To avoid a database shutdown, execute a database-wide VACUUM in "mydb".

> (A manual VACUUM should fix the problem, as suggested by the hint; but
> note that the VACUUM must be performed by a superuser, else it will fail to
> process system catalogs and thus not be able to advance the database's
> datfrozenxid.)

It occurs to me to wonder how this scenario will interact with the
recent changes to let VACUUM skip pages. AFAIR there is not a way for a
manual VACUUM to set the anti-wraparound mode, is there? Perhaps manual
VACUUMs should not have the page skip behavior at all?

regards, tom lane