Kill: Display error on invalid UTF-8 rather than list of signals

[iburaky2]

Fixes #12816

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/pid-pipe (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/cp/link-heap is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.