Add a Reboot resource into core. (RFC code.)

For ease of testing, the actual reboot is commented out.
should be `shutdown -r now` and on Solaris you want `shutdown -i6 -g0 -y` (although i think sol11 now supports `shutdown -r now` as well?  but we need to support sol9).  the former is actually BSD-ish while the latter is SysV-ish.  Linux picked the BSD-style.