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.

src/systemctl/systemctl.h

@@ -77,6 +77,8 @@ extern UnitFilePresetMode arg_preset_mode;
 extern char **arg_wall;
 extern const char *arg_kill_whom;
 extern int arg_signal;
+extern int arg_kill_value;
+extern bool arg_kill_value_set;
 extern char *arg_root;
 extern usec_t arg_when;
 extern const char *arg_reboot_argument;