ovs-vsctl: Allow command-specific options to mingle with global options.

ovs-vsctl: Allow command-specific options to mingle with global options.

Until now, a command like "ovs-vsctl --may-exist add-br br0" yielded a
confusing error message.  Users had to realize that the correct form was
"ovs-vsctl -- --may-exist add-br br0", but instead they often reported a
bug or gave up in frustration.  Even though the behavior was documented, it
was counterintuitive.

This commit allows command-specific options to be mixed with global
options, making both forms of the command listed above equally acceptable.

CC: 691508@bugs.debian.org
Reported-by: Adam Heath <doogie@brainfood.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>