safekeeper: correctly handle signals

[skyzh]

Problem

errno is not preserved in the signal handler. This pull request fixes it. Maybe related: #6969, but does not fix the flaky test problem.

Summary of changes

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

2700 tests run: 2569 passed, 0 failed, 131 skipped (full report)

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Flaky tests (2)

Postgres 15

• test_compute_pageserver_connection_stress: debug

Postgres 14

• test_sharding_split_smoke: debug

Code coverage* (full report)

• functions: 28.3% (7128 of 25166 functions)
• lines: 46.8% (43693 of 93285 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
b62ad06 at 2024-03-18T19:57:28.015Z :recycle:}

[petuhovskiy]

LGTM, but I'm not sure if it affects anything.

[skyzh]

waiting for #7170

[arssher]

but I'm not sure if it affects anything.

Same, I don't think it will fix the original issue.