Makes it possible to calculate it as part of the signature.

See
http://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html
for rationale.

Not used yet.

https://forums.aws.amazon.com/thread.jspa?messageID=340398 explains
why: a HEAD request to a location-specific bucket will get a 301
redirect that doesn't include enough info to actually redirect. The
intent is to prompt the client to use the vhost. So use a vhost
pre-emptively on HEAD requests.

Include a list of current valid locations and update the link to
Amazon documentation on the topic.

First, don't try to bulk-delete an empty vector of objects.

Second, in addition to the existing too-many check, error out on
too-few arguments to bulk-delete.