This convenience was necessary to be used for golang library functions like io.Copy and io.Pipe
where we shouldn't be writing proxies and alternatives returning *probe.Error
This change also brings more changes across code base for clear separation regarding where an error
interface should be passed encapsulating *probe.Error and where it should be used as is.
- Explicitly cast statfs_t members to int64 (this structure is
platform-specific)
- Add pass-through New methods to Darwin SHA package
- Move scsi pkg types to common translation unit (package was empty)
- Add stub implementations mount/disk ops for OS X
