s3util: automatically increase part size

This enables uploading any size object (up to the
Amazon-imposed maximum of 5TiB) while doing as
little buffering as possible for smaller objects.
be sure part size doesn't overflow
test vhost bucket derivation
test with literal Service definition

User may have copied out the definition of DefaultService.
To keep compatibility, the definition as written here must
continue to work.
Support signing for third-party services.