core: add `io.systemd.Manager.{PowerOff,Reboot,SoftReboot,Halt,Kexec}`

core: add `io.systemd.Manager.{PowerOff,Reboot,SoftReboot,Halt,Kexec}`

This adds the low-level io.systemd.Manager shutdown support. This
is (much) simpler than the logind one. It mimics dbus but uses
a shared helper for the simple cases.

Note that this is more restrictive than the dbus version. The
dbus version uses SD_BUS_VTABLE_CAPABILITY(CAP_SYS_BOOT) but
the varlink version uses varlink_check_privileged_peer(link).
This is mostly because I'm not sure how to do the equivalent
in a race-free way.

Thanks to Daan for suggesting this.