lock: Don't copy the lock when checking for process existence

The lock test creates a lock and checks that it is not stale. This also
tests whether the corresponding process still exists. However, it is
possible that the lock is refreshed concurrently, which updates the lock
timestamp. Calling `processExists()` with a value receiver, however,
creates an unsynchronized copy of this field. Thus call the method using
a pointer receiver.

internal/restic/lock_unix.go

@@ -29,7 +29,7 @@ func uidGidInt(u *user.User) (uid, gid uint32, err error) {
 // checkProcess will check if the process retaining the lock
 // exists and responds to SIGHUP signal.
 // Returns true if the process exists and responds.
-func (l Lock) processExists() bool {
+func (l *Lock) processExists() bool {
 	proc, err := os.FindProcess(l.PID)
 	if err != nil {
 		debug.Log("error searching for process %d: %v\n", l.PID, err)