Further revise r322327 and r322352 in release/packages/kernel.ucl.

Use PPID and PID to kill off the pre-install and parent pkg(8)
processes unless 'Y' or 'y' are entered at the prompt if the user
wants to proceed with upgrading the kernel and userland at the same
time.

This restores some of the logic and intent of r322327, with the
caveat of printing "child process terminated unexpectedly."

MFC after: 5 days
MFC with: r322327, r322352
Sponsored by: The FreeBSD Foundation