Compare install and running kernel on FreeBSD

Summary:
When running freebsd-update, a new kernel can be installed.

Add a monitoring check to reboot the server in such case.

Thanks to #gcu members for code review.

Test Plan:

• apply unit roles/core/monitoring
• /usr/local/share/monitoring/checks/nrpe/check-kernel.sh

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2648