pcap_io: introduce init_once helper that is called with priviledges

When using netsniff-ng with dropping priviledges, we have to introduce
another pcap helper function that is called once before we drop the
priviledges. In this function we have to invoke the disc I/O scheduler
policy, because it needs priviledges. Otherwise netsniff-ng will fail
with "Failed to set io prio for pid" on startup, since we're not root
anymore.

Reported-by: Doug Burks <doug.burks@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>