All `net/rpc` requests go to `/minio`, so the existing
generic handler for reserved bucket check would essentially
erroneously send errors leading to distributed setups to
wait infinitely.
For `net/rpc` requests alone we should skip this check and
allow resource bucket names to be from `/minio` .
startOffset was re-assigned to '0' so it would end up
copying wrong content ignoring the requested startOffset.
This also fixes the corruption issue we observed while
using docker registry.
Fixes https://github.com/docker/distribution/issues/2205
Also fixes#3842 - incorrect routing.
Avoid passing size = -1 to PutObject API by requiring content-length
header in POST request (as AWS S3 does) and in Upload web handler.
Post handler is modified to completely store multipart file to know
its size before sending it to PutObject().
