struct sockaddr_un

Struct sockaddr_un is used in program for bind() to listen on a local file.

Path is one of its attributes and only has 108-byte size. It may happen
that the path is too small to contain the whole real path, which contains
current working directory now. This may lead to two sockets listening on
the same file.

So, we change the path to be "/tmp". It ensures that filename cannot
exceed the limit.

Additionally, to run it well with namespace, make new directory named
'tmp' under constructed root.
Signed-off-by: Yicheng Qin <yichengq@google.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>