uptime: add support for OpenBSD using utmp

[lcheylus]

• Add OpenBSD support for uptime using utmp-classic crate to parse UTMP file (/var/run/utmp by default): get uptime and count of logged users.
• Enable tests for uptime (previously disabled on OpenBSD) and add a new specific test for OpenBSD (parse UTMP file, same as used for users test).

Fix #5664

---

Build and tests OK on OpenBSD current/amd64 and Linux Debian/testing with Rust 1.79.0

• Unit tests:

  $ cargo test -v --no-default-features --features uptime
  (...)
  test test_uptime::test_failed ... ok
  test test_uptime::test_invalid_arg ... ok
  test test_uptime::test_uptime ... ok
  test test_uptime::test_uptime_check_users_openbsd ... ok
  test test_uptime::test_uptime_with_dir ... ok
  test test_uptime::test_uptime_since ... ok
  test test_uptime::test_uptime_with_extra_argument ... ok
  test test_uptime::test_uptime_with_non_existent_file ... ok
  
  test result: ok. 34 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.19s

• Test for uptime --since

  $ target/debug/coreutils uptime -s
  2024-06-29 08:57:30

• Comparison with GNU uptime (binary = guptime on OpenBSD, installed via coreutils-9.5 package)

  $ target/debug/coreutils uptime && guptime
  17:57:50  up 1 day,  9:00,  1 user,  load average: 0.07, 0.16, 0.14
  17:57:50  up 1 day,  9:00,  1 user,  load average: 0.07, 0.16, 0.14

[sylvestre]

Thanks!