package/iputils: use capabilities if possible

package/iputils: use capabilities if possible

If support for extended attributes is enabled, then we can use them to
store capabilities. If not, we keep using the setuid bit.

arping does not get a capability, as it can be used for arp poisoning.

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
[yann.morin.1998@free.fr:
  - resort to using q full-fledged conditional block
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>