freebsd-update: Add check for kernel modules

People get confused when some software (VirtualBox, etc) does not work as
expected (or at all) after a major upgrade.

We have a nice way to deal with this when using sources, namely including
PORTS_MODULES in /etc/make.conf, but we lack something similar for binary
updates.

This patch retrieves a list of kernel modules installed from packages and
advises the user to recompile from ports to avoid problems.

Approved by: zlei@
Differential Revision: https://reviews.freebsd.org/D39695