Implement GET NUMLOGINS

Fixes #11

nut-client/examples/blocking.rs

@@ -34,6 +34,7 @@ fn main() -> nut_client::Result<()> {
     for (name, description) in conn.list_ups()? {
         println!("\t- Name: {}", name);
         println!("\t  Description: {}", description);
+        println!("\t  Number of logins: {}", conn.get_num_logins(&name)?);
 
         // Get list of mutable variables
         let mutable_vars = conn.list_mutable_vars(&name)?;