`os.File` offers a `Chmod()` method. This is often safer and more direct
to use than `os.Chmod()` because it operates on an open file descriptor
rather than having to lookup the file by name. Without this, it's
possible for the target file to be renamed, in which case an
`os.Chmod()` would either fail or apply to any file that's taken its
place.
Therefore, add the `Chmod()` method to the `File` interface, and
implement it for all `File` implementations. The bulk of this change is
in `MemMapFs`, which required moving the chmod functionality down into
the `mem` package so that it can be shared between both `mem.File` and
`MemMapFs`.
Before, the OpenFile method was unimplemented, and just returned `nil,
nil`. Because the Afero.TempFile method calls the OpenFile method, this
behavior caused a nil pointer exception when TempFile was used with
sftpfs.
This commit adds a simple implementation for sftpfs.OpenFile, fixing the
exception thrown when using it with TempFile.
Oliver Bone