Revise part of r322327 in release/packages/kernel.ucl.

It appears I misunderstand process forking and signal handling in
how the pre-/post-install scripts are executed internally by pkg(8).
In some cases (not all), ^C when prompted to cancel the kernel
package update will stop the pre-install script from executing, but
allow pkg(8) to continue extracting the package when it is not the
intent.

In order to keep somewhat of an anti-footshooting measure in place,
print the recommendation to install the kernel package first if
ASSUME_ALWAYS_YES is false and TERM is set, then sleep for 5 seconds
to allow the user to see the message.

MFC after: 5 days
MFC with: r322327
X-MFC-Note: Maybe not until I am happy with this..
Sponsored by: The FreeBSD Foundation