socket-util-unix: Fix umask race in bind_unix_socket().

The umask is a process-wide value, so bind_unix_socket() races with file
creation in other Open vSwitch threads.  This fixes the race.

The workaround for non-Linux systems is not ideal, but I do not know any
other general solution.  I tested the workaround only on Linux.

CC: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Flavio Leitner <fbl@redhat.com>