ovs-ctl: Don't decrease max open fds if already set higher

A user may set LimitNOFILE through systemd or other means to set
the maximum number of open file descriptors. Only modify the ulimit
if not already set to a higher value by the user.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Gospodarek <gospo@redhat.com>