net-dns/pihole: implement autoupdate of macvendor.db

[Bogdan107]

May be added some commented cronjob, which update macvendor.db once per month? ebuild may contain ewarn or einfo message with See /etc/cron.d/pihole to manual activation of automatic update of macvendor.db file.

[Tatsh]

I need to know how often macvendor.db changes. Does Pi-hole in its normal installation download macvendor.db often?

[Tatsh]

Also note for me, implement:

• [x] systemd timer for updating gravity.db
• [x] systemd timer/crontab for updating macvendor.db (if we do it)