Looking at PR lib/34744, I was not sure if we expect userland to set
sun_len correctly. A quick grep through our tree shows about 10 out of about
20 source files doing it, the others seem to leave sun_len as zero.

The kernel cares about sun_len at very few places, and I fail to see the
big picture. Since apparently the userland parts that do not set sun_len
work just fine, I guess we could replace the few kernel usages of
sun_len with SUN_LEN()?

Maybe we could even get rid of the sun_len field completely? (But then it
might not be worth the compmatibility issues - and of course I might just
be missing something.)

Martin