add systemd socket support

[rhjdvsgsgks]

get fd from systemd to make systemd socket works

any1/wayvnc#146 need this

[any1]

I think that it would be better to have a more generic interface that doesn't depend on systemd. We could have a function called something like struct nvnc* nvnc_listen(int fd). That would also open the doors for adding all sorts of sockets without having to add them to the library.