Add Chmod() method to File

[owbone]

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.

[CLAassistant]

All committers have signed the CLA.