utils: format_duration_human: calculate years too

utils: format_duration_human: calculate years too

Add years and skip showing minute scale once we got over a year,
that's just not relevant anymore at that scale..

Months are just not an ideal thing to show, as they have different
lengths too (leap years have similar issue, but they differ 0.27%
from a normal year, while shortest to longest month is 10.7%
difference!)

Weeks could be done though, they're fixed at 7 days, but for now I
want to avoid unwieldy day numbers like 2634 d as that's just hard to
frame correctly. Also adding years now does not makes adding weeks in
the future impossible anyway..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>