pid1: use initrd.target in the initramfs by default

pid1: use initrd.target in the initramfs by default

This makes the code do what the documentation says. The code had no inkling
about initrd.target, so I think this change is fairly risky. As a fallback,
default.target will be loaded, so initramfses which relied on current behaviour
will still work, as along as they don't have a different initrd.target.

In an initramfs created with recent dracut:
$ ls -l usr/lib/systemd/system/{default.target,initrd.target}
lrwxrwxrwx. usr/lib/systemd/system/default.target -> initrd.target
-rw-r--r--. usr/lib/systemd/system/initrd.target
So at least for dracut, there should be no difference.

Also avoid a pointless allocation.