servers: fix to reverse SA_RESTART option for sigaction() on modern codepath

[vszakats]

Historically servers used the deprecated siginterrupt() function to
configure restart behavior on specific signals. It accepts a flag, where
1 means to remove the SA_RESTART option, and 0 means to enable it.

In year 2021 3fb6e5a introduced the
modern alternative to the codebase, replacing siginterrupt() with
sigaction(). After this patch, supporting, modern, systems reacted on
the same flag, but, by accident, set the SA_RESTART bit when flag is
1, and did not set it when 0. This reversed the previous behavior, and
the one still used on the siginterrupt() legacy codepath.

Fix it by reversing the SA_RESTART logic for the sigaction()
codepath, syncing it with the pre-existing behavior.

I find it odd this did not cause any perceivable issue for 5 years, even
though it's the active one in most Unix envs.

Spotted by GitHub Code Quality, though suggesting to fix
siginterrupt() calls. But looking into the history, those were correct
all along.

Refs:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/siginterrupt.html
https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigaction.html
https://www.man7.org/linux/man-pages/man3/siginterrupt.3.html
https://www.man7.org/linux/man-pages/man2/sigaction.2.html

Follow-up to 3fb6e5a #6529

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• try dropping signal handling from the servers. → servers: try dropping custom signal handlers #22485

[Copilot]

Pull request overview

This PR fixes a long-standing behavioral mismatch in the curl test server signal handling: the modern sigaction() path had SA_RESTART behavior reversed relative to the historical siginterrupt() semantics, and this change restores the original meaning of the flag.

Changes:

• Reverse SA_RESTART handling in set_signal() when using sigaction() so it matches the legacy siginterrupt() behavior.

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[vszakats]

I plan to merge this as-is, soon, then try yanking the signal handling in
a follow-up PR. (then, if successful, remove the unused configure bits)

[vszakats]

augment review

[vszakats]

@aisle-analyzer

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[vszakats]

augment review
@aisle-analyzer

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[augmentcode]

🤖 Augment PR Summary

Summary: This PR fixes the inverted restart behavior for server signal handlers on the modern sigaction() codepath.

Changes:

• Adjusts SA_RESTART flag handling so the runtime behavior matches historical siginterrupt() semantics.
• Renames the set_signal() flag to norestart and switches it to an int to reflect the underlying APIs.
• Updates all call sites to pass explicit 0/1 values for restart vs. no-restart behavior.

Technical Notes: The intent is to keep sigaction() and legacy siginterrupt() paths consistent (1 disables restart, 0 enables it).

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[augmentcode]

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[vszakats]

Well, this may not be the last word said about signals and restart behavior.

Just spotted this commit:
9d0d828

Which sets SIGARLM, to the reverse of how it's set later on. Quite confusing.
I'm thinking maybe deleting the legacy interface would be a step to make this
easier to digest.