Add --signature-v2 and signature_v2 config option

For use with S3-like systems that don't yet support V4 signatures.

If you use --signature-v2 against an AWS endpoint that only supports
v4 signatures, s3cmd will loop forever trying to re-issue the same
(failing) request with signature v2.  This isn't ideal, but is a side
effect of the attempt to upgrade from V2 to V4 silently.