MFC r308139,r308157,r308158:

r308139 (by bapt):

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

Relnotes: yes

r308157 (by bapt):

Fix typo in cron(8) date

r308158 (by bapt):

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