Use cgroups.AddProc() for cgroups v1

All occurrences only passed a PID, so we can use this utility to make
the code more symmetrical with their cgroups v2 counterparts.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

services/server/server_linux.go

@@ -56,9 +56,7 @@ func apply(ctx context.Context, config *srvconfig.Config) error {
 					return err
 				}
 			}
-			if err := cg.Add(cgroups.Process{
-				Pid: os.Getpid(),
-			}); err != nil {
+			if err := cg.AddProc(uint64(os.Getpid())); err != nil {
 				return err
 			}
 		}