Add support in uucore for OpenBSD

[lcheylus]

• uucore/src/lib/features/fs.rs: add target_os = OpenBSD when needed

• uucore/src/lib/features/fsext.rs: implement FsUsage::new for OpenBSD

• fixes uucore build fails on OpenBSD #5448

• initial code by n1000 https://github.com/n1000/coreutils/tree/openbsd_compile_fixes

---

Build OK with cargo build --release on OpenBSD-current/amd64 and Rust 1.74.0

$ target/release/coreutils
coreutils 0.0.23 (multi-call binary)

Usage: coreutils [function [arguments...]]

Currently defined functions:

    [, b2sum, b3sum, base32, base64, basename, basenc, cat, cksum, comm, cp, csplit, cut, date,
    dd, df, dir, dircolors, dirname, du, echo, env, expand, expr, factor, false, fmt, fold,
    hashsum, head, join, link, ln, ls, md5sum, mkdir, mktemp, more, mv, nl, numfmt, od, paste,
    pr, printenv, printf, ptx, pwd, readlink, realpath, rm, rmdir, seq, sha1sum, sha224sum,
    sha256sum, sha3-224sum, sha3-256sum, sha3-384sum, sha3-512sum, sha384sum, sha3sum,
    sha512sum, shake128sum, shake256sum, shred, shuf, sleep, sort, split, sum, tac, tail, tee,
    test, touch, tr, true, truncate, tsort, unexpand, uniq, unlink, vdir, wc, yes

Tests results with cargo --test

(...)
failures:
    test_cat::test_error_loop
    test_cp::test_copy_contents_fifo
    test_cp::test_cp_parents_2_dirs
    test_cp::test_cp_preserve_xattr
    test_cp::test_no_preserve_mode
    test_cp::test_preserve_hardlink_attributes_in_directory
    test_cp::test_preserve_mode
    test_du::test_du_basics_subdir
    test_du::test_du_d_flag
    test_du::test_du_dereference
    test_du::test_du_hard_link
    test_du::test_du_no_dereference
    test_du::test_du_one_file_system
    test_du::test_du_soft_link
    test_du::test_du_symlink_multiple_fail
    test_du::test_du_threshold
    test_ls::test_ls_allocation_size
    test_ls::test_ls_human_si
    test_ls::test_ls_long_total_size
    test_split::test_dev_zero
    test_split::test_filter_broken_pipe
    test_split::test_number_by_bytes_dev_zero
    test_tail::test_follow_descriptor_vs_rename1
    test_tail::test_follow_descriptor_vs_rename2
    test_tail::test_follow_name_move1
    test_tail::test_follow_name_move2
    test_tail::test_follow_name_move_create1
    test_tail::test_follow_name_move_create2
    test_tail::test_follow_name_move_retry1
    test_tail::test_follow_name_move_retry2
    test_tail::test_follow_name_retry_headers
    test_tail::test_follow_when_files_are_pointing_to_same_relative_file_and_file_stays_same_size
    test_tail::test_retry3
    test_tail::test_retry4
    test_tail::test_retry5
    test_tail::test_retry7
    test_tail::test_retry8
    test_tail::test_retry9
    test_tail::test_stdin_redirect_dir
    test_test::test_file_is_sticky
    test_test::test_file_owned_by_egid
    test_touch::test_touch_changes_time_of_file_in_stdout
    test_touch::test_touch_dash

test result: FAILED. 2429 passed; 43 failed; 30 ignored; 0 measured; 0 filtered out; finished in 236.55s

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/inotify-dir-recreate. tests/tail/inotify-dir-recreate is passing on 'main'. Maybe you have to rebase?

[cakebaker]

Thanks for your PR!