lib/privs: Don't use CAP_NET_BROADCAST

lib/privs: Don't use CAP_NET_BROADCAST

From what I can tell, CAP_NET_BROADCAST has never been required for any
functionality in the Linux kernel, so we do not really need it.

However, it causes breakage in contexts where Quagga is started with a
limited set of capabilities, e.g. in Docker, because these may not
include CAP_NET_BROADCAST and in the case of Docker do not even support
adding CAP_NET_BROADCAST.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>