Since both `NetworkManager.service` and `NetworkManager-initrd.service` are
allocated for the same bus name (`org.freedesktop.NetworkManager`) and this is
not allowed, the best option is to use a systemd generator to install them only
in the initrd, instead of setting fixed Install sections.
Fixes#1814
Decouple from a specific initrd generator (dracut) the systemd services that
provide networking in the initrd using NM, thus allowing other systemd-based
initrd generators to take advantage of it.
These new services are:
- `NetworkManager-config-initrd.service`: it starts very early at boot, parses
the kernel command line using `nm-initrd-generator` and sets the
`/run/NetworkManager/initrd/neednet` flag to activate the other initrd services,
and also sets the hostname if needed.
- `NetworkManager-initrd.service`: it basically does the same job as the
`NetworkManager.service`, but in the initrd.
- `NetworkManager-wait-online-initrd.service`: ordered before
`network-online.target`, it will allow other services that require networking to
delay their start until NM has finished.
