Add _ping user/group.

OK natano on a previous diff which used a different uid/gid.
naddy@ pointed out that uid/gid was already taken on "important" systems.
Turns out we cannot easily recycle freed up uids/gids so settle on 51.

src/etc/group

@@ -21,6 +21,7 @@ _fingerd:*:33:
 _sshagnt:*:34:
 _x11:*:35:
 utmp:*:45:
+_ping:*:51:
 _rebound:*:52:
 _unbound:*:53:
 _dpb:*:54:

src/etc/master.passwd

@@ -9,6 +9,7 @@ _rstatd:*:30:30::0:0:rpc.rstatd:/var/empty:/sbin/nologin
 _rusersd:*:32:32::0:0:rpc.rusersd:/var/empty:/sbin/nologin
 _fingerd:*:33:33::0:0:fingerd:/var/empty:/sbin/nologin
 _x11:*:35:35::0:0:X Server:/var/empty:/sbin/nologin
+_ping:*:51:51::0:0:ping privdrop user:/var/empty:/sbin/nologin
 _rebound:*:52:52::0:0:Rebound DNS Daemon:/var/empty:/sbin/nologin
 _unbound:*:53:53:unbound:0:0:Unbound Daemon:/var/unbound:/sbin/nologin
 _dpb:*:54:54::0:0:dpb privsep:/var/empty:/sbin/nologin