Add SUM sysctl

[Warren Block]

On Sat, 16 Apr 2011, dieterbsd at engineer.com wrote:

> 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.

Another option: kern.hostid is set nonzero pretty early in the rc 
scripts.