shared/hibernate-util: drop support for kernels lacking /sys/power/resume_offset

shared/hibernate-util: drop support for kernels lacking /sys/power/resume_offset

The current fallback path is actually unreliable, given
the kernel *supports* setting the resume offset through
cmdline after all, but just not exposed under /sys/.
For v258 let's drop it hence.

I didn't bump the baseline to 4.17, but merely documented
new requirement in README, because there's certainly more
compat stuff to drop between 4.3 and 4.17, and README is
a useful list for things to kill. We'll get to 5.4 eventually.