[BUGFIX] Only create one keypair in rsaauth

If there are two login forms on one page the second form's private
key overwrites the first form's private key so the first form doesn't
work. With this patch only one keypair gets created and it doesn't
matter how many login forms you have one one page.

Change-Id: I42660140aea72d1888cc73d56e83b823206a0797
Fixes: #24877
Fixes: #6708
Releases: 6.2, 6.1, 6.0, 4.5
Reviewed-on: https://review.typo3.org/10485
Reviewed-by: Stefan Neufeind
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack
Reviewed-by: Daniel Gorges
Tested-by: Daniel Gorges
Reviewed-by: Markus Klein
Tested-by: Markus Klein