inform user when using default encryption key

Currently if you generate a default encryption key:
`proxmox-backup-client key create --kdf none`

all backup operations which don't explicitly disable encryption will be
encrypted with this key.

I found it quite surprising, that my backups were all encrypted without
me explicitly specfying neither key nor encryption mode

This patch informs the user when the default key is used (and no
crypt-mode is provided explicitly)

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>

src/bin/proxmox-backup-client.rs

@@ -817,7 +817,10 @@ fn keyfile_parameters(param: &Value) -> Result<(Option<Vec<u8>>, CryptMode), Err
     Ok(match (keydata, crypt_mode) {
         // no parameters:
         (None, None) => match key::read_optional_default_encryption_key()? {
-            Some(key) => (Some(key), CryptMode::Encrypt),
+            Some(key) => {
+                println!("Encrypting with default encryption key!");
+                (Some(key), CryptMode::Encrypt)
+            },
             None => (None, CryptMode::None),
         },