seat: preserve current VT on transient update failures and fix errno semantics for free-VT lookup

[Copilot]

This PR addresses the remaining review feedback in thread pullrequestreview-4409920047 for the new control-socket VT APIs. The updates are scoped to seatd/seat.c and align behavior with existing VT-state assumptions.

• Keep cur_vt stable on VT refresh failures

  • In seat_update_vt(), failures from terminal_open(0) or invalid results from terminal_current_vt() no longer overwrite seat->cur_vt.
  • Errors are still propagated via -1/errno, but transient query failures no longer corrupt seat state used by VT signal paths.

• Preserve accurate errno behavior in seat_find_available_vt()

  • errno is now restored only on failure (vt == -1), not unconditionally.
  • This avoids success paths returning with stale non-zero errno.

Example of the errno behavior fix:

int vt = terminal_find_available(tty0fd);
int saved_errno = errno;
close(tty0fd);
if (vt == -1) {
	errno = saved_errno;
}
return vt;

[deepin-ci-robot]

Hi @Copilot. Thanks for your PR.

I'm waiting for a linuxdeepin member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Copilot

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[zccrs]

用不到了

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