Add SUM sysctl

[dieterbsd at engineer.com]

> once you reboot into SUM to install world, you are doomed, BECAUSE
> ...
> Kernel will bitch (GELI part), about world->kernel mismatch and you
> won't be able to install world as you cant decrypt geom providers!!

Suggestion 1:  Install the new stuff into different disk partition(s),
leaving the partition(s) you are currently running alone.  Then if
something doesn't work and the new installation doesn't boot you
are not doomed, you can simply boot the previous partition(s) again.

Suggestion 2:  The kernel may not have an official flag for single
vs multi user mode but you can fake it.  Try something like
"pgrep syslogd".  If syslogd is running assume multiuser mode.  If
syslogd is not running assume single user mode.