server: Fix fake "Address already in use" error

In the current workflow, socket file will be deleted if it already exists.
However, if the socket file is a symbolic link and the file that it refers
to doesn't exist, we will got "Address already in use" because bind()
thinks the socket file exists and won't create it.

Now, use lstat() to determine whether the socket file exists.

Signed-off-by: Liu Wenlong <liuwl.fnst@cn.fujitsu.com>