units: add initrd-cryptsetup.target

units: add initrd-cryptsetup.target

For encrypted block devices that we need to unlock from the initramfs,
we currently rely on dracut shipping `cryptsetup.target`. This works,
but doesn't cover the case where the encrypted block device requires
networking (i.e. the `remote-cryptsetup.target` version). That target
however is traditionally dynamically enabled.

Instead, let's rework things here by adding a `initrd-cryptsetup.target`
specifically for initramfs encrypted block device setup. This plays the
role of both `cryptsetup.target` and `remote-cryptsetup.target` in the
initramfs.

Then, adapt `systemd-cryptsetup-generator` to hook all generated
services to this new unit when running from the initrd. This is
analogous to `systemd-fstab-generator` hooking all mounts to
`initrd-fs.target`, regardless of whether they're network-backed or not.