kill: implement an option for trigger kernel oom killer

As mentioned in issue https://github.com/rfjakob/earlyoom/issues/358 and README, after Linux 5.17, OOM sysrq will always kill a process. This implements an option for trigger the kernel oom killer i.e. write an `f` to `/proc/sysrq-trigger`. Since the victim cannot be chosen by earlyoom, some of the features about customized victim selection won't take effect.