logind: Fix org.freedesktop.login1.set-reboot-to-boot-loader-menu saving to...

logind: Fix org.freedesktop.login1.set-reboot-to-boot-loader-menu saving to the wrong file in the non EFI case

According to the docs, and to the
org.freedesktop.login1.get-reboot-to-boot-loader-menu code, the
(oneshot) boot-loader-menu timeout should be stored in
/run/systemd/reboot-to-boot-loader-menu, but the set method was storing it
in /run/systemd/reboot-to-loader-menu.

This commit fixes this. Note that the fixed name also is a better match
for the dbus call names and matches the related
/run/systemd/reboot-to-boot-loader-entry structure, so fixing the set code,
rather then the get code + docs seems like the right thing to do here.