Make initrd.target the default target in the initrd

First, rename root-fs.target to initrd-root-fs.target to clarify its usage.

Mount units with "x-initrd-rootfs.mount" are now ordered before
initrd-root-fs.target. As we sometimes construct /sysroot mounts in
/etc/fstab in the initrd, we want these to be mounted before the
initrd-root-fs.target is active.

initrd.target can be the default target in the initrd.

                             (normal startup)
                                    :
                                    :
                                    v
                              basic.target
                                    |
             ______________________/|
            /                       |
            |                  sysroot.mount
            |                       |
            |                       v
            |             initrd-root-fs.target
            |                       |
            |                       v
            |            initrd-parse-etc.service
(custom initrd services)            |
            |                       v
            |            (sysroot-usr.mount and
            |             various mounts marked
            |               with fstab option
            |                x-initrd.mount)
            |                       |
            |                       v
            |                initrd-fs.target
            |                       |
            \______________________ |
                                   \|
                                    v
                               initrd.target
                                    |
                                    v
                          initrd-cleanup.service
                               isolates to
                         initrd-switch-root.target
                                    |
                                    v
             ______________________/|
            /                       |
            |        initrd-udevadm-cleanup-db.service
            |                       |
(custom initrd services)            |
            |                       |
            \______________________ |
                                   \|
                                    v
                        initrd-switch-root.target
                                    |
                                    v
                        initrd-switch-root.service
                                    |
                                    v
                               switch-root