cron(8): convert vfork() usage to fork()
vfork() is error-prone, and the usage here definitely grew to not be
clearly OK given vfork-semantics; e.g. setusercontext(3) within the child.
Rip out vfork() and the rest of the references to it. fork is heavier, but
it's unclear that the difference will be all that obvious.
Reported by: Andrew Gierth and sigsys@gmail.com