UCT/TCP: Use SIOCGIFCONF ioctl when /sys/class/net is missing.

[civodul]

What

This change provides alternative code that uses the SIOCGIFCONF ioctl to get the names of the available TCP network interfaces.

Why ?

In some cases such as isolated build environments (as found in GNU Guix), containers, or non-Linux based system, /sys is missing.

How ?

Using the old, portable SIOCGIFCONF ioctl.

It may be that the SIOCGIFCONF can in fact replace the /sys-based code since the information returned should be the same. WDYT?