[jdk17u-dev] RFR: 8293466: libjsig should ignore non-modifying sigaction calls

[Thomas Stuefe]

Small fix that fixes corner cases with signal chaining in libjsig.

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Commit messages:
 - Backport b1ed40a87ab357d1b51ac5102bba181f21ffa9b6

Changes: https://git.openjdk.org/jdk17u-dev/pull/1049/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1049&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293466
  Stats: 22 lines in 1 file changed: 14 ins; 3 del; 5 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1049.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/1049/head:pull/1049

PR: https://git.openjdk.org/jdk17u-dev/pull/1049