timeout: add --verbose to diagnose timeouts

timeout: add --verbose to diagnose timeouts

This is useful as handling in shell is complicated
with the varying exit status in the --kill-after case.

* src/timeout.c (main): Handle '-v' and store
COMMAND for the diagnostic.
(cleanup): Diagnose the signal name before sending.
(usage): Document -v, --verbose.
* doc/coreutils.texi (timeout invocation): Likewise.
* tests/misc/timeout.sh: Add a test case.
* NEWS: Mention the new feature
Fixes https://bugs.gnu.org/21760