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_windows.go

@@ -0,0 +1,31 @@
+// +build windows
+
+package fserrors
+
+import (
+	"syscall"
+)
+
+const (
+	WSAECONNABORTED   syscall.Errno = 10053
+	WSAHOST_NOT_FOUND syscall.Errno = 11001
+	WSATRY_AGAIN      syscall.Errno = 11002
+	WSAENETRESET      syscall.Errno = 10052
+	WSAETIMEDOUT      syscall.Errno = 10060
+)
+
+func init() {
+	// append some lower level errors since the standardized ones
+	// don't seem to happen
+	retriableErrors = append(retriableErrors,
+		syscall.WSAECONNRESET,
+		WSAECONNABORTED,
+		WSAHOST_NOT_FOUND,
+		WSATRY_AGAIN,
+		WSAENETRESET,
+		WSAETIMEDOUT,
+		syscall.ERROR_HANDLE_EOF,
+		syscall.ERROR_NETNAME_DELETED,
+		syscall.ERROR_BROKEN_PIPE,
+	)
+}