cron(8): add support for /etc/cron.d and /usr/local/etc/cron.d

For automation tools it is way easier to maintain files in directories rather
than modifying /etc/crontab.

The files in those directories are in the same format as /etc/crontab

Reviewed by: adrian
MFC after: 2 weeks
Relnotes: yes
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D8400