Changes:
- added a vkernel_bin rc.conf var which sets the vkernel binary for all
vkernels that don't specify their own (usually most/all vkernels use the same
binary, so thought this would be useful)
- the root img is no longer required for vkernels (e.g. diskless vkernels)
- stopping vkernels requires a pidfile (it doesn't kill by binary name
anymore) and after a set timeout (vkernel_kill_timeout) it does a SIGKILL if
the vkernel has not shut down properly yet
- when starting a vkernel, do not just check if the pidfile exists, but also
if a process with that pid already exists
- when starting a vkernel, also redirect STDIN due to some strange behaviour
when not redirecting it (e.g. after the vkernel paniced, the arrow keys
stopped functioning ... daemon(8) does not by default redirect STDIN so I
guess the vkernel could still influence the controlling console?)
