RFR: JDK-8289910: unify os::message_box across posix platforms

[Matthias Baesken]

The method os::message_box can be unified across the posix platforms.

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Commit messages:
 - JDK-8289910

Changes: https://git.openjdk.org/jdk/pull/9409/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9409&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289910
  Stats: 77 lines in 4 files changed: 20 ins; 57 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9409.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9409/head:pull/9409

PR: https://git.openjdk.org/jdk/pull/9409