src: add support for UID/GID socket info

With this patch libnetfilter_queue is able to parse UID/GID
socket information.

Signed-off-by: Valentina Giusti <Valentina.Giusti@bmw-carit.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
utils: add example usage of nfq_get_uid and nfq_get_gid

[ Mangled this patch to indicate that this kernel does not support
  UID/GID retrieval not to confuse users --pablo ]

Signed-off-by: Valentina Giusti <Valentina.Giusti@bmw-carit.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>