Manta is an Object Storage by [Joyent](https://www.joyent.com/)
This PR adds initial support for Manta. It is intended as non-production
ready so that feedback can be obtained.
This check incorrectly rejects most valid filenames. The only filenames Sia
forbids are leading forward slashes and path traversal characters, but it's
better to simply allow Sia to reject invalid names on its own rather than try
to anticipate errors from Sia:
https://github.com/NebulousLabs/Sia/blob/master/doc/api/Renter.md#path-parameters-4
Paul Stack