synchroniser : fix broken synchronisation

Append was used instead of extend which made a list inside of a list
instead of appending the content at the end fix that.
Convert backup.maxbackups to str as needed for subprocess.

src/unisync/synchroniser.py

@@ -141,11 +141,11 @@ class Synchroniser:
             else:
                 other.append("Name *")
 
-            other.append([
+            other.extend([
                 "-backuploc",
                 backup.location,
                 "-maxbackups",
-                backup.max_backups,
+                str(backup.max_backups),
                 "-backupsuffix",
                 backup.backupsuffix,
                 "-backupprefix",