systemctl: make sure the kernel is loaded before kexec'ing

systemctl: make sure the kernel is loaded before kexec'ing

We just load the same kernel that would be loaded by default by sd-boot, with
the same options. Changing the kernel or initramfs or options is left for
later.

Now we will refuse to continue if loading fails. This makes 'systemctl kexec'
more predictable: it will not fall back to normal reboot if the kernel is
not loaded.