sd-rtnl: extend tests a bit
networkd: add basic VLAN support
networkd: improve logging a bit

Fix/add some structured logging messages, and be uniform about when we WARN and ERR.
dbus1-generator: add missing "path=" and "unix:*" to DBUS environment variables

This works around the goa daemon crash:
  Jan 25 20:54:24 lon goa[1363]: goa-daemon version 3.10.2 starting [main.c:117, main()]
  Jan 25 20:54:24 lon systemd[424]: Starting Legacy D-Bus Protocol Compatibility Daemon (PID 1363/UID 2702)...
  Jan 25 20:54:24 lon systemd[424]: Started Legacy D-Bus Protocol Compatibility Daemon (PID 1363/UID 2702).
  Jan 25 20:54:24 lon kernel: goa-daemon[1363]: segfault at 20 ip 00007f46914b26d5 sp 00007fff1ae6d9a0 error 4 in libtelepathy-glib.so.0.80.1[7f469144f000+228000]
  Jan 25 20:54:24 lon systemd-coredump[1368]: Process 1363 (goa-daemon) dumped core.
  Jan 25 20:54:32 lon goa[1375]: goa-daemon version 3.10.2 starting [main.c:117, main()]