Break the fs package up into smaller parts.

The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.

The new code layout is documented in CONTRIBUTING.

fs/fserrors/retriable_errors.go

@@ -0,0 +1,21 @@
+// +build !plan9
+
+package fserrors
+
+import (
+	"syscall"
+)
+
+func init() {
+	retriableErrors = append(retriableErrors,
+		syscall.EPIPE,
+		syscall.ETIMEDOUT,
+		syscall.ECONNREFUSED,
+		syscall.EHOSTDOWN,
+		syscall.EHOSTUNREACH,
+		syscall.ECONNABORTED,
+		syscall.EAGAIN,
+		syscall.EWOULDBLOCK,
+		syscall.ECONNRESET,
+	)
+}