install: don't trim the root away when creating leading dirs

[MsfPablo]

Fixes #13232.

For install -D src /dest, the parent to create is /. The trailing-separator loop stripped separators unconditionally, and since / is nothing but a separator it came out as an empty path. "".exists() is false, so install decided the parent was missing, tried to create it, and failed:

$ install -D -m755 file /file
install: /file: chmod failed with error No such file or directory (os error 2)
install: failed to chmod '/file'

The loop now stops while one separator is left. / stays /, which exists and is a directory, so the copy proceeds — matching the documented -D behaviour, where the only leading component of /file is a root that already exists.

I pulled the trimming into a trim_trailing_separators helper so the root boundary is unit-testable, and switched its byte conversion to uucore::os_str_from_bytes; the inline version used OsStrExt::from_bytes, which is behind a #[cfg(unix)] import in this file.

Verified against the reproducer: before, the chmod ENOENT above; after, install gets far enough to report the real reason for the destination being unwritable, and the source is left alone.

Tests: unit tests for the helper, covering /, //, ordinary trailing slashes, and the no-op cases. The end-to-end case needs a writable /, so I did not add an integration test for it — the existing 104 install tests still pass.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/rm/isatty (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.

[sylvestre]

Please add a test in test_install.rs