getmypid() can be disabled on shared hosts causing backups to fail

On some web hosts, `getmypid()` can be disabled which causes backups to fail by incorrectly reporting the backup process has been killed.

In `save_backup_process_id()`, use of `getmypid()` when it's not available means an empty string is stored to the backup process ID file.

Then, in `is_backup_still_running()` the following comparison returns true because empty is falsy:

```
if ( false == ( $process_id = $this->get_backup_process_id() ) )
			return false;
```

We don't ever use the backup process ID, so we might be able to remove it entirely.