motd: Fix Questions List address

Following mailing lists migration from Mailman to Mlmmj.

https://lists.FreeBSD.org/mailman/listinfo/freebsd-questions/
-->
https://lists.freebsd.org/subscription/freebsd-questions

Adding a short link (as we already have for Faq and Handbook), so all
links can be indented the same way.

Reviewed by: imp (earlier version)
Approved by: lwhsu
Differential Revision: https://reviews.freebsd.org/D33101

Add more explicit instructions about updating motd

Not that you can regenerate the motd by editing motd.template and
running 'service motd restart' rather than rebooting.

Small wordsmithing by me, and updated the example from FreeBSD 2.1.6.1
release to 12.1 release.

Submitted by: Dan Mack

motd: Generate from template to /var/run

Update login(1), its manual pages, similar utilities, and motd.5 to refer to
the new location.

Suggested by: delphij@ (re: r349256)
Reviewed by: bcr (manpages), delphij
Differential Revision: https://reviews.freebsd.org/D20721