[BUGFIX] Prevent indexing of Backend login page in search engines

[BUGFIX] Prevent indexing of Backend login page in search engines

Change-Id: Id22f34fd39daf33c69f233a54e6d969e4bab1eae
Resolves: #22599
Releases: 4.6, 4.5
Reviewed-on: http://review.typo3.org/3453
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers