Copy errors & fs packages from the restic repo and fix import paths

Tedious, but I see no better way...

fs/file_nonlinux.go

@@ -0,0 +1,15 @@
+// +build !linux !go1.4
+
+package fs
+
+import "os"
+
+// Open opens a file for reading.
+func Open(name string) (File, error) {
+	return os.OpenFile(fixpath(name), os.O_RDONLY, 0)
+}
+
+// ClearCache syncs and then removes the file's content from the OS cache.
+func ClearCache(f File) error {
+	return nil
+}