[UNIX]: Extend unix_sysctl_(un)register prototypes

Add the struct net * argument to both of them to use in
the future. Also make the register one return an error code.

It is useless right now, but will make the future patches
much simpler.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>