pstream-unix: Increase listen count to 64 in punix_open().

In my test with openstack setup, ovs-ofctl executes failed when there are
many flow rules to be added by multiple threads.
The error like this:
ovs-ofctl: /var/run/openvswitch/br1.mgmt: failed to open socket (Protocol
error)

In the function listen(fd, 10) in punix_open(), the number 10 should be
modified to more bigger, such as 64 maybe a proper value.

Signed-off-by: Lilijun <jerry.lilijun@huawei.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>