lib: posix: sleep() should report unslept time in seconds

In the case that `sleep()` is interrupted, the POSIX spec requires
it to return the number of "unslept" seconds (i.e. the number of
seconds requested minus the number of seconds actually slept).

Since `k_sleep()` already returns the amount of "unslept" time
in ms, we can simply use that.

Signed-off-by: Chris Friedt <cfriedt@meta.com>