mutable writev should be able to delete shares

[warner]

A call to testv_and_readv_and_writev() which specifies that the new share length = 0 should be interpreted as meaning that the share should be deleted outright.

This would give the mutable publish/repair code the ability to properly migrate shares to better servers (to accomplish rebalancing), first by writing a new share to the new server, then by removing the old share from the old server.

This doesn't increase the writer's authority at all: if they can fill the share with zeros, or truncate it down to zero bytes, they can do just as much damage as deleting the share would do.

[warner]

Done, in 13a3ef5ec1f651cc.