Fix bug in VM buddylist algorithm

When returning pages back to their buddylist's freelist, make
sure to adjust the index into the busymap by the order of page.
Stop treating zero-length IO vectors as an "out of memory" condition

There's no particular reason that the message-passing infrastructure
should refuse to send a message that contains zero parts. That's
no different than a traditional non-vectored message with zero length,
which we already allow.
Merge branch 'master' into heap