bsdinstall: Add virtual_oss service option

Since virtual_oss is now part of base, there is no reason not to provide
an installer option to enable it, and make it more visible to new users,
who might also benefit from the devd rules in /etc/devd/snd.conf, which
use virtual_oss, as well as 8532b4a436 ("rc: virtual_oss: Create a
loopback device in the default configuration").

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Reviewed by:	ivy
Pull-Request:	https://ron-dev.freebsd.org/FreeBSD/src/pulls/31

usr.sbin/bsdinstall/scripts/services

@@ -71,6 +71,11 @@ if [ -x "${BSDINSTALL_CHROOT}/etc/rc.d/moused" ]; then
 	    moused \"PS/2 mouse pointer on console\" ${moused_enable:-off}"
 fi
 
+if [ -x "${BSDINSTALL_CHROOT}/etc/rc.d/virtual_oss" ]; then
+	DAEMON_OPTIONS="$DAEMON_OPTIONS \
+	    virtual_oss \"Sound server\" ${virtual_oss_enable:-off}"
+fi
+
 exec 5>&1
 DAEMONS=$(eval bsddialog --backtitle \"$OSNAME Installer\" \
     --title \"System Configuration\" --no-cancel --separate-output \