vt: Disable bell by default

Bell is either useless if you're working on remote servers or really annoying
when you're working with a local machine that have a loud buzzer.
Switch the default to have it disable.

Reviewed by: imp, pstef, tsoome
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D32543