systemctl: add --kill-value= argument to systemctl

systemctl: add --kill-value= argument to systemctl

This allows accompanying a signal with a value (as supported for Linux
Realtime signals). This is particularly useful as it allows us to do
stuff like this:

   systemctl kill --kill-whom=main --kill-value=0x300 systemd-journald

In order to ask journald to flush its allocation caches and compact
memory.