cron: add log suppression and mail suppression for successful runs

This commit adds two new extensions to crontab, ported from OpenBSD:

• -n: suppress mail on succesful run
• -q: suppress logging of command execution

The -q option appears decades old, but -n is relatively new. The
original proposal by Job Snijder can be found here [1], and gives very
convincing reasons for inclusion in base.

This patch is a nearly identical port of OpenBSD cron for -q and -n
features. It is written to follow existing conventions and style of the
existing codebase.

Example usage:

1. should only send email, but won't show up in log
2. * * * * -q date

1. should not send email
2. * * * * -n date

1. should not send email or log
2. * * * * -n -q date

1. should send email because of ping failure
2. * * * * -n -q ping -c 1 5.5.5.5

[1]: https://marc.info/?l=openbsd-tech&m=152874866117948&w=2

PR: 237538
Submitted by: Naveen Nathan <freebsd_t.lastninja.net>
Reviewed by: bcr (manpages)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20046