[BUGFIX] ext:rsaauth cliBackend pollutes cli output

The CommandLineBackend using exec "openssl genrsa" outputs its key
generation to standard error. If SAPI is cli (eg. in unit tests),
this is is shown to the user. The patch silences openssl by
redirecting stderr to /dev/null (NUL on Windows systems).

Resolves: #58530
Related: #51436
Releases: 6.2
Change-Id: I702d2d3180bc2e32e5548a4402d4eefb02dd2523
Reviewed-on: https://review.typo3.org/29877
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Reviewed-by: Markus Klein
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes