[BUGFIX] sessionTimeout for BE does not work

Changes an is_string() check to a check out of t3lib_Utility_Math
which will also accept '86400' as a valid value and not only 86400.

Change-Id: I5a8bfe4b1015d9a6195a9831272bd7e5518110dc
Resolves: #32274
Releases: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/7079
Reviewed-by: Markus Klein
Reviewed-by: Simon Schaufelberger
Tested-by: Simon Schaufelberger
Reviewed-by: Michael Stucki
Reviewed-by: Oliver Klee
Tested-by: Markus Klein
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer