Add extra logging in xtrabackup binary around suspend file

Some customer is having very strange issue with suspend file detection where innobackupex 'sees' a suspend file before it is actually created by xtrabackup, causing innobackupex to begin copying/streaming non-InnoDB files before xtrabackup is actually finished. This could possibly be caused by multiple instances of backup running with the same tmpdir or there could be some other yet to be discovered issue. It is very difficult to debug this as there isn't any testing for an existing suspend file in xtrabackup nor does it report when it is creating one. We should add a little extra validation and logging just to make it easier to attempt to debug these customer issues.