bug#34488: Add sort --limit, or document workarounds for sort|head error messages

[Pádraig Brady]

Thanks for doing all that.
I've attached a few changes:

- spelling fixes
- usage() clarified/reordered
- ensure sigset_t are initialized
- Don't setprocmask() unless specified
- Simplified SETMASK_SIGNAL_OPTION handling
- The test missed `env` as a prerequisite
- The test was slow/spun cpu, so used sleep instead of seq
- Used $SHELL in case sh didn't support trap

I see that the last signal that `kill -l` lists is not included.
I think we should be processing SIGNUM_BOUND also?

cheers,
Pádraig

env-signal-fixes.patch
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