virtual_oss: Introduce virtual_oss_default_control_device rc variable

The -t option gives the user the ability to create a control device for
a given virtual_oss(8) instance, so that the instance's configuration
can be manipulated during runtime with virtual_oss_cmd(8). As is
expected, the control device's name is not known, since it is specified
by the user.

This patch introduces a virtual_oss_default_control_device rc variable,
which defaults to "vdsp.ctl". The goal of this is that third-party
programs and scripts can access the control device of the default
virtual_oss(8) configuration without guessing. This is especially useful
for sbin/devd/snd.conf which deals with hot-swapping sound devices using
virtual_oss(8).

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55670