core: replace OnFailureIsolate= setting by a more generic OnFailureJobMode= setting and make use of it where applicable

units: initrd, multi-user - add a few more words to Description=

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