ovs-ctl: Exit, instead of resuming, after handling fatal signals.

ovs-ctl: Exit, instead of resuming, after handling fatal signals.

When I wrote the "trap" calls in ovs-ctl, I had the mistaken notion that
"trap $cmd $signal" would execute $cmd and then exit when $signal was
caught.  This is incorrect.  Instead, it executes $cmd and then resumes
executing the shell script.

On the other hand, "trap $cmd 0" does by itself what I wanted: it causes
the shell to execute $cmd and then exits due to the signal.  So this commit
changes the offending traps to use this form.

Bug #14290.
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>