`uptime`: uucore/get_uptime should return a `Duration`

[drinkcat]

uucore's get_uptime function:

• Returns a signed integer value (but the uptime can't be a negative value?)
• Ignores fractional parts of a second even though the Linux kernel provides that in /proc/uptime.

Possibly depending on the fractional timestamp value at boot time, the second point can cause a 1 second offset between GNU coreutils and uutils:

$ uptime --since
2025-04-23 18:26:36
$ cargo run -p uu_uptime -- --since
2025-04-23 18:26:37

I believe we should change the function signature to return a Duration.

Not sure how much we care about semver in uucore, we would need to modify procps/uu_top as well: see #7289.

[sylvestre]

it is fine if we need to update procps, it is WIP