zebra: raise the privileges before calling socket()

zebra: raise the privileges before calling socket()

Because of recent changes when creating AF_NETLINK socket, kernel will
cache capabilities of the caller and if file descriptor is used or
otherwise handed to another process it will check that current user has
necessary capabilities to use the socket. Hence we need to ensure we
have necessary capabilities when creating the socket and at the time we
use the socket.

See: http://www.spinics.net/lists/netdev/msg280198.html

Signed-off-by: Michal Sekletar <msekleta@redhat.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>