This change brings the storagedriver API in line with the Go standard library's
use of int64 for offsets. The main benefit is simplicity in interfacing with
the io library reducing the number of type conversions in simple code.
Fixes/tests listing for keys beginning with "/"
No longer extraneously wraps Closers in ioutil.NopClosers
Uses omitempty for all ipc struct type fields
This only works for a specific whitelist of error types, which is
currently all errors in the storagedriver package.
Also improves storagedriver tests to enforce proper error types are
returned
