os: implement os.(*File).WriteAt (#3697)

os: implement os.(*File).WriteAt

Signed-off-by: Achille Roussel <achille.roussel@gmail.com>

src/os/filesystem.go

@@ -58,6 +58,9 @@ type FileHandle interface {
 	// Write writes up to len(b) bytes to the file.
 	Write(b []byte) (n int, err error)
 
+	// WriteAt writes b to the file at the given absolute offset
+	WriteAt(b []byte, offset int64) (n int, err error)
+
 	// Close closes the file, making it unusable for further writes.
 	Close() (err error)
 }