units: order systemd-fsck@.service after local-fs-pre.target.

With this change, it becomes possible to order a unit to activate before any
modifications to the file systems. This is especially useful for supporting
resume from hibernation.
hibernate-resume: add a tool to write a device node's major:minor to /sys/power/resume.

This can be used to initiate a resume from hibernation by path to a swap
device containing the hibernation image.

The respective templated unit is also added. It is instantiated using
path to the desired resume device.
hibernate-resume-generator: add a generator for instantiating the resume unit.

hibernate-resume-generator understands resume= kernel command line parameter
and instantiates the systemd-resume@.service accordingly if it is passed.

This enables resume from hibernation using device specified on the kernel
command line, and it may be specified either as "/dev/disk/by-foo/bar"
or "FOO=bar", not only "/dev/sdXY" which is understood by the in-kernel
implementation.

So now resume= is brought on par with root= in terms of possible ways to
specify a device.