Save CSRs that were generated in PEM format with suffix .pem

This ensures that CSRs in PEM format are saved with suffix .pem and that
CSRs in binary format are saved with suffix .p10. Previously, Kleopatra
generated the CSRs in PEM format (see previous commit) and saved them
with suffix .p10.

(cherry picked from commit 98c152b1b823c92f200c26238dcb86d6b4e3469a)

• GnuPG-bug-id: T8115