zbeacon: uses only interfaces that are running

Adds a check in s_get_interface to ignore the interfaces that do not
have the IFF_UP flag set.
zbeacon: Ignore interfaces that are bonding slaves

Ignore interfaces that have the IFF_SLAVE flat set, if this flag is
defined.
Merge pull request #559 from ahhentz/to_pull

zbeacon: uses only interfaces that are running