[jdk11u-dev] RFR: 8293466: libjsig should ignore non-modifying sigaction calls

[Goetz Lindenmaier]

A small fix for 11.

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Commit messages:
 - Backport b1ed40a87ab357d1b51ac5102bba181f21ffa9b6

Changes: https://git.openjdk.org/jdk11u-dev/pull/2158/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2158&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/jdk11u-dev/pull/2158.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2158/head:pull/2158

PR: https://git.openjdk.org/jdk11u-dev/pull/2158