uucore: Disallow slashes in determine_backup_suffix

[aweinstock314]

Comparative output from GNU ln and uu ln prior to the fix:

root@b082d601e194:/tmp# ln --version
ln (GNU coreutils) 9.7
Packaged by Debian (9.7-3)
Copyright (C) 2025 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Parker and David MacKenzie.
root@b082d601e194:/tmp# touch a b
root@b082d601e194:/tmp# mkdir b_
root@b082d601e194:/tmp# SIMPLE_BACKUP_SUFFIX=_/../c ln -b -s a b
root@b082d601e194:/tmp# ls -l
total 4
-rw-r--r-- 1 root root    0 Feb 25 23:54 a
lrwxrwxrwx 1 root root    1 Feb 25 23:55 b -> a
drwxr-xr-x 2 root root 4096 Feb 25 23:54 b_
-rw-r--r-- 1 root root    0 Feb 25 23:54 b~

root@03081849be59:/tmp# ln --version
ln (uutils coreutils) 0.6.0
root@03081849be59:/tmp# touch a b
root@03081849be59:/tmp# mkdir b_
root@03081849be59:/tmp# SIMPLE_BACKUP_SUFFIX=_/../c ln -b -s a b
root@03081849be59:/tmp# ls -l
total 4
-rw-r--r-- 1 root root    0 Feb 25 23:53 a
lrwxrwxrwx 1 root root    1 Feb 25 23:54 b -> a
drwxr-xr-x 2 root root 4096 Feb 25 23:53 b_
-rw-r--r-- 1 root root    0 Feb 25 23:53 c

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[codspeed-hq]

Merging this PR will improve performance by 5.76%

⚡ 1 improved benchmark
✅ 293 untouched benchmarks
⏩ 42 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	shuf_input_range[1000000]	89.7 ms	84.8 ms	+5.76%

---

_{Comparing Zellic:backupcontrol-no-path-traversal (ae6ecb1) with main (a611f98)}

Footnotes

1. 42 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩