timeout: add --preserve-status to always propagate the exit status

timeout: add --preserve-status to always propagate the exit status

It's useful for commands that support running for an indeterminite
amount of time, to not return a specific timeout exit status (124),
and instead let the command handle the timeout signal and return
a status for the work done so far.

* doc/coreutils.texi (timeout invocation): Describe the new option.
* src/timeout.c (preserve_status): A new global boolean to
enable the --preserve-status behavior.
(usage): Describe the new option.
(main): Don't return EXIT_TIMEOUT of preserve_status is set.
* tests/misc/timeout.sh: Add a test for the new option.