Fix lxc-netstat -- argument processing

Fix lxc-netstat -- argument processing

Commit 21e487f2 introduced the use of getopt, but getopt will fail when
it sees arguments meant for netstat that are not in [short|long]options.
There should not be any ambiguity about arguments with the same letter:
those to the left of the -- are destined for lxc-netstat and those to
the right for the real netstat, which the original code handles by
shifting out all arguments it recognizes before the -- is hit.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>