timeout: honor ignored signal dispositions

This behavior was depended on in our trap_sigpipe_or_skip_ helper,
and now that we're handling all terminating signals, we should
consistently honor their ignored signal dispositions.

* NEWS: Mention the change in behavior, especially in regard
to shell background jobs.
* src/timeout.c (sig_needs_handling): A new helper that return TRUE,
for --signal, SIG_ALRM, or non ignored signals.
(cleanup_install): Filter handled signals with the helper.
(block_cleanup_and_chld): Likewise.
* tests/timeout/timeout-group.sh: Adjust to use the now required
`env --default-signal=...` wrapper to reset (auto) ignored signals.
Also change the termination signal from SIGINT to SIGUSR1
to generalize the test signals not specially handled by the shell,
and newly handled by timeout(1).
* tests/timeout/timeout.sh: Add a test case for SIGPIPE
to ensure the ignored signal disposition is honored.

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