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.