systemd has a behavior that is very nice for containers.  If you send it
SIGRTMIN+3 (37 on some Linux systems), it will halt the machine (see
https://www.freedesktop.org/software/systemd/man/systemd.html).
Further, if the env var "container" is set, systemd will actually exit.

9001-systemd-sigrtmin3-compat.diff adds this functionality to runit.
Prior to this, runit would of course just sit and spin after entering
stage 3 (its halt/shutdown/reboot target).  Now if the env var
"container" is set, runit will exit.  Plus runit can be signaled by
Docker (or whatever container manager) to shutdown immediately via
SIGRTMIN+3.

(Also, this commit adds the custom "emulab" bit into the package version
strings.)