rc: virtual_oss: Create a loopback device in the default configuration

The loopback device allows us to record desktop sound by reading from
it, or even use it as an input device, for example during a call.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Reviewed by:	emaste
Pull-Request:	https://ron-dev.freebsd.org/FreeBSD/src/pulls/16

libexec/rc/rc.d/virtual_oss

@@ -39,6 +39,7 @@ virtual_oss_default_args="\
 	-i 8 \
 	-f /dev/dsp${default_unit} \
 	-d dsp \
+	-l dsp.loop \
 	-t ${virtual_oss_default_control_device}"
 
 # Set to NO by default. Set it to "YES" to enable virtual_oss.