[io] Prevent losing data due to SIGTERM (under Unix)

[ferdymercury]

This Pull request:

Changes or fixes:

Fixes https://github.com/root-project/root/issues/13300

• I tested this locally on Ubu22 and it works fine.
• I do not know how to properly test it using GTEST, I only found test_polling.cxx doing something like that but it looked too complicated, so I ended up simplifying.
• I do not know if Windows must catch SIGTERM somewhere else @bellenot ?

Checklist:

• [x] tested changes locally
• [x] updated the docs (if necessary)

[github-actions[bot]]

Test Results

    22 files      22 suites   3d 18h 35m 56s ⏱️  3 707 tests  3 660 ✅ 0 💤 47 ❌ 79 603 runs  79 509 ✅ 0 💤 94 ❌

For more details on these failures, see this check.

Results for commit 64374c4e.

:recycle: This comment has been updated with latest results.

[bellenot]

@ferdymercury The SIGILL and SIGTERM signals aren't generated under Windows. They're included for ANSI compatibility. See the signal reference

[dpiparo]

Let's have a look to the tests once again