This method gets called when a writable file gets closed in the
filesystem.  This allows filesystems to react to changed file sizes
due to writes.

Adds the following methods to fuse.File:

	GetAttr() *Attr
	Utimens(atimeNs uint64, mtimeNs uint64) Status
	Truncate(size uint64) Status
	Chown(uid uint32, gid uint32) Status
	Chmod(perms uint32) Status

Largely untested.

This allows new files to be edited directly.