nixos/systemd-tmpfiles: add initrd support

This adds support for declaring tmpfiles rules exclusively for the
systemd initrd. Configuration is possible through the new option
`boot.initrd.systemd.tmpfiles.settings` that shares the same interface as
`systemd.tmpfiles.settings`.

I did intentionally not replicate the `rules` interface here, given that
the settings attribute set is more versatile than the list of strings
used for `rules`. This should also make it unnecessary to implement the
workaround from 1a68e21d474c5d6005812459c9bce28168625384 again.

A self-contained `tmpfiles.d` directory is generated from the new initrd
settings and it is added to the initrd as a content path at
`/etc/tmpfiles.d`.

The stage-1 `systemd-tmpfiles-setup.service` is now altered to no longer
operate under the `/sysroot` prefix, because the `/sysroot` hierarchy
cannot be expected to be available when the default upstream service is
started.

To handle files under `/sysroot` a slightly altered version of the
upstream default service is introduced. This new unit
`systemd-tmpfiles-setup-sysroot.service` operates only under the
`/sysroot` prefix and it is ordered between `initrd-fs.target` and the
nixos activation.

Config related to tmpfiles was moved from initrd.nix to tmpfiles.nix.

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