Handle Unix domain socket connections from outside our namespace

Handle Unix domain socket connections from outside our namespace

NixOS uses Unix domain sockets for certain host <-> container
interaction; i.e. the host connects to a socket visible in the
container's directory tree, where the container uses a .socket unit to
spawn the handler program on demand. This worked in systemd 203, but
in 212 fails with "foo.socket failed to queue service startup job
(Maybe the service file is missing or not a template unit?): No data
available".

The reason is that getpeercred() now returns ENODATA if it can't get
the PID of the client, which happens in this case because the client
is not in the same PID namespace. Since getpeercred() is only used to
generate the instance name, this patch simply handles ENODATA by
creating an instance name "<nr>-unknown".

[zj: reorder clauses and remove (unsigned long) casts.]