Add audio group

To be initially used by virtual_oss(8) loopback devices.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Reviewed by:	emaste
Pull-Request:	https://ron-dev.freebsd.org/FreeBSD/src/pulls/26

etc/group

@@ -15,6 +15,7 @@ sshd:*:22:
 smmsp:*:25:
 mailnull:*:26:
 guest:*:31:
+audio:*:43:
 video:*:44:
 realtime:*:47:
 idletime:*:48:

sys/sys/conf.h

@@ -155,6 +155,7 @@ typedef int dumper_hdr_t(struct dumperinfo *di, struct kerneldumpheader *kdh);
 #define		GID_OPERATOR	5
 #define		GID_BIN		7
 #define		GID_GAMES	13
+#define		GID_AUDIO	43
 #define		GID_VIDEO	44
 #define		GID_RT_PRIO	47
 #define		GID_ID_PRIO	48