MFC r308139, r308157-r308158

r308139:
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

r308157:
Fix typo in cron(8) date

Reported by: jilles

r308158:
Allow symlinks to be followed in cron.d directories and fix detection of
regular files on NFS

Reported by: jilles