[FEATURE] Raise user authentication log severity

[FEATURE] Raise user authentication log severity

Raise the syslog severity for user authentication failures from 0
(info) to 2 (warning). Log scanners like 'fail2ban' can be used to
detect login errors without setting the syslog severity level to 'info'.

Change-Id: Id08f71ef281631326c4b4a75a62088876604c82d
Resolves: #36979
Releases: 6.0
Reviewed-on: http://review.typo3.org/11517
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn