environment: append unit_id to error messages regarding EnvironmentFile
nspawn: fix DeviceAllow list

Commit 864e17068 ("nspawn: actually allow access to /dev/net/tun in the
container") added "/dev/net/tun" to the list of allowed devices but forgot
to tweak the array length, which caused "/dev/kdbus/*" to be missed.
systemd: continue switch-root even if umount fails

Leaving the old root around seems better than aborting the
switch.
systemd: log deserialization errors as warnings

If we failed to parse something that we wrote ourselves,
things are seriously off. This is also likely to lead to
problems futher on.
systemd: try harder to bind to notify socket

Without the socket open we are going to crash and burn. If for
whatever reason we fail during deserialization we will fail when
trying to open the socket. In this case it is better to unlink the old
socket and maybe lose some messages, than to continue without the
notification socket.

Of course this situation should not happen, but we should handle
it as gracefully as possible anyway.

https://bugzilla.redhat.com/show_bug.cgi?id=1099299