It's not true that I'm a time traveller visiting from the future.
start_process_worker() -- fork pkg(8).  Leave the child to carry on
and do whatever is required as normal.  However iff the child exits
with status EX_NEEDRESTART, fork again and replay the same process.
Rather than execve()'ing a second process when it is detected that pkg
needs to be re-run, fork the process early-on, let the forked child
run, and do whatever wrk is required and signal back to the parent by
setting its exit status if it needs to be restarted.

This allows restart of pkg(8) when used in a jail or chroot.

Fixes #746