This is a fixup for c1ae82f448.
nix' `passAsFile` does not create empty files for variables that are
`null`.
This results in the following error for units that have no overrides or
content, but are, e.g. `wantedBy`:
`mv: cannot stat '': No such file or directory`.
Minimal reproducer:
`systemd.units.empty.wantedBy = [ "multi-user.target" ];`
This is often necessary when a unit is loaded in via `systemd.packages`.
According to systemd.netdev manpage:
```
MACAddress=
Specifies the MAC address to use for the device, or takes the special value "none". When "none", systemd-networkd does not request the MAC address for
the device, and the kernel will assign a random MAC address. For "tun", "tap", or "l2tp" devices, the MACAddress= setting in the [NetDev] section is
not supported and will be ignored. Please specify it in the [Link] section of the corresponding systemd.network(5) file. If this option is not set,
"vlan" device inherits the MAC address of the master interface. For other kind of netdevs, if this option is not set, then the MAC address is
generated based on the interface name and the machine-id(5).
Note, even if "none" is specified, systemd-udevd will assign the persistent MAC address for the device, as 99-default.link has
MACAddressPolicy=persistent. So, it is also necessary to create a custom .link file for the device, if the MAC address assignment is not desired.
```
Therefore, `none` is an acceptable value.
This change is made for two reasons:
1. If `toString config.restartTriggers` containes `\n`, systemd unit
file will be ill-formed.
2. This change can limit length of the trigger, although it doesn't
matter in most cases.
This patch allows creation of files like
/etc/systemd/system/user-.slice.d/limits.conf with
systemd.units."user-.slice.d/limits.conf" = {
text = ''
[Slice]
CPUAccounting=yes
CPUQuota=50%
'';
};
which previously threw an error
Also renames the systemd-unit-path test to sytsemd-misc, and extends it to
test that `systemd.units` can handle directories. In this case we make
sure that resource limits specified in user slices apply.
