gobind: add Put
gobind: add Put

- A simplified Client.Put so that Android can upload files (CL for that
  is forthcoming).

No tests affected. No new lint or vet errors.

Change-Id: I1fccd289ce8d2704d9a070e0bd70f54bd2b82137
Reviewed-on: https://upspin-review.googlesource.com/2640
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>

upspin/upspin,upspin/upspin