crypt: add --crypt-server-side-across-configs flag

This can be used for changing filename encryption mode without
re-uploading data.

See: https://forum.rclone.org/t/revert-filename-encryption-method/17454/

backend/crypt/crypt.go

@@ -72,6 +72,20 @@ NB If filename_encryption is "off" then this option will do nothing.`,
 			Name:       "password2",
 			Help:       "Password or pass phrase for salt. Optional but recommended.\nShould be different to the previous password.",
 			IsPassword: true,
+		}, {
+			Name:    "server_side_across_configs",
+			Default: false,
+			Help: `Allow server side operations (eg copy) to work across different crypt configs.
+
+Normally this option is not what you want, but if you have two crypts
+pointing to the same backend you can use it.
+
+This can be used, for example, to change file name encryption type
+without re-uploading all the data. Just make two crypt backends
+pointing to two different directories with the single changed
+parameter and use rclone move to move the files between the crypt
+remotes.`,
+			Advanced: true,
 		}, {
 			Name: "show_mapping",
 			Help: `For all files listed show how the names encrypt.
@@ -181,6 +195,7 @@ func NewFs(name, rpath string, m configmap.Mapper) (fs.Fs, error) {
 		CanHaveEmptyDirectories: true,
 		SetTier:                 true,
 		GetTier:                 true,
+		ServerSideAcrossConfigs: opt.ServerSideAcrossConfigs,
 	}).Fill(f).Mask(wrappedFs).WrapsFs(f, wrappedFs)
 
 	return f, err
@@ -193,6 +208,7 @@ type Options struct {
 	DirectoryNameEncryption bool   `config:"directory_name_encryption"`
 	Password                string `config:"password"`
 	Password2               string `config:"password2"`
+	ServerSideAcrossConfigs bool   `config:"server_side_across_configs"`
 	ShowMapping             bool   `config:"show_mapping"`
 }