ovs-ctl: Raise the limit on the number of open file descriptors.

Since the removal of dispatcher thread, OVS creates 'n-handler-threads'
file descriptors for each bridge port.  To allow more bridge ports
be supported, this commit raises the limit on the number of open file
descriptors from 7500 to 65535.

Bug #1254038.

Signed-off-by: Alex Wang <alexw@nicira.com>