Number of network interfaces is limited

The number of network interfaces is now limited by a constant that is now set to 32. If more than 32 network interfaces (either physical or virtual) are found, xllmnrd will abort with a syslog message.

It should not be limited by a fixed number, of course, but one of the reason that prevent it is the lack of generic container types in C. If I would have started the work it in C++, it would be much easier to resolve this issue, but I decided to go with C99.