cgls: mangle user-provided unit names

so the CLI interface is now similar to `systemctl`, i.e. if no unit name
suffix is provided, assume `.service`.

Fixes: #20492

Before:
```
$ systemd-cgls --unit user@1000
Failed to query unit control group path: Invalid argument
Failed to list cgroup tree: Invalid argument
```

After:
```
$ build/systemd-cgls --unit user@1000
Unit user@1000.service (/user.slice/user-1000.slice/user@1000.service):
├─session.slice (#4939)
│ ├─pipewire-pulse.service (#5203)
│ │ └─7711 /usr/bin/pipewire-pulse
...
```

1 files changed, 8 insertions, 4 deletions

diff --git a/src/cgls/cgls.c b/src/cgls/cgls.c
index 775bd84ad2..936ea4d3af 100644
--- a/src/cgls/cgls.c
+++ b/src/cgls/cgls.c
@@ -208,7 +208,11 @@ static int run(int argc, char *argv[]) {
 
                         if (arg_show_unit != SHOW_UNIT_NONE) {
                                 /* Command line arguments are unit names */
-                                _cleanup_free_ char *cgroup = NULL;
+                                _cleanup_free_ char *cgroup = NULL, *unit_name = NULL;
+
+                                r = unit_name_mangle(*name, UNIT_NAME_MANGLE_WARN, &unit_name);
+                                if (r < 0)
+                                        return log_error_errno(r, "Failed to mangle unit name: %m");
 
                                 if (!bus) {
                                         /* Connect to the bus only if necessary */
@@ -219,16 +223,16 @@ static int run(int argc, char *argv[]) {
                                                 return bus_log_connect_error(r, BUS_TRANSPORT_LOCAL);
                                 }
 
-                                q = show_cgroup_get_unit_path_and_warn(bus, *name, &cgroup);
+                                q = show_cgroup_get_unit_path_and_warn(bus, unit_name, &cgroup);
                                 if (q < 0)
                                         goto failed;
 
                                 if (isempty(cgroup)) {
-                                        q = log_warning_errno(SYNTHETIC_ERRNO(ENOENT), "Unit %s not found.", *name);
+                                        q = log_warning_errno(SYNTHETIC_ERRNO(ENOENT), "Unit %s not found.", unit_name);
                                         goto failed;
                                 }
 
-                                printf("Unit %s (%s):\n", *name, cgroup);
+                                printf("Unit %s (%s):\n", unit_name, cgroup);
                                 fflush(stdout);
 
                                 q = show_cgroup_by_path(cgroup, NULL, 0, arg_output_flags);