backup: use proxmox-systemd crate

Some systemd code got split out from proxmox-sys and left there
re-exported with a deprecation marker, use the newer crate, the
workspace already depends on proxmox-systemd anyway.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/tools/systemd/unit.rs

@@ -99,7 +99,7 @@ pub fn reload_unit(unit: &str) -> Result<(), Error> {
 #[test]
 fn test_escape_unit() -> Result<(), Error> {
     fn test_escape(i: &str, expected: &str, is_path: bool) {
-        use proxmox_sys::systemd::{escape_unit, unescape_unit};
+        use proxmox_systemd::{escape_unit, unescape_unit};
 
         let escaped = escape_unit(i, is_path);
         assert_eq!(escaped, expected);