fixed setting unix socket permissions

first call fchmod() to prevent TOCTTOU, then apply permissions using
chmod()

src/net/SocketUtil.hxx

@@ -32,6 +32,10 @@ class SocketAddress;
 /**
  * Creates a socket listening on the specified address.  This is a
  * shortcut for socket(), bind() and listen().
+ * When a unix socket is created (domain == AF_UNIX), its
+ * permissions will be stripped down to prevent unauthorized
+ * access. The caller is responsible to apply proper permissions
+ * at a later point.
  *
  * Throws #std::system_error on error.
  *