Following [PR#5620](https://github.com/uutils/coreutils/pull/5620) `uptime` tool is not supported on OpenBSD, due to the lack of support for utmpx (see https://github.com/uutils/coreutils/issues/5596). => implement `uptime` on OpenBSD with an alternate solution without using utmpx.
