devd: autofs: Move autofs related events to a separate file

If a user don't have FreeBSD-autofs installed there is no need to try calling
automount on every GEOM event.
It's also easier to add/delete autofs related event in a separate file.

Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D42495
Reviewed by: imp