Allow vt(4) to disable terminal bell with `sysctl kern.vt.bell_enable=0`,
similar as syscons(4) do.

Submitted by:	Tiwei Bie <btw@mail.ustc.edu.cn>
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272715 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f