RFR: JDK-8274320: os::fork_and_exec() should be using posix_spawn

[Thomas Stuefe]

Hi, may I have reviews for this small patch please?

`os::fork_and_exec()`, used in the hotspot to spawn child programs (scripts etc) in error situations, should be using `posix_spawn()`. 

ATM it uses either `fork()` or `vfork()`. `vfork()` got deprecated on MacOS and we get build errors (JDK-8274293) - even though in this case it would be completely fine to use. This leaves us with `fork()` for MacOS, which has the known problems with large-footprint-parents. This matters here especially since we also use os::fork_and_exec to implement `-XX:OnError` for OOM situations.

We already use posix_spawn() as default for Runtime.exec() since JDK 15, and it is available on all our Unices. We also should use it here.

I kept the name of the function (fork_and_exec) since people know it, even though it's more incorrect now than before.

Tests:
- manual tests using -XX:OnError with various scripts, including checking that env variables are passed correctly
- manually ran runtime/ErrorHandling tests
- GHAs

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Commit messages:
 - Merge branch 'master' into JDK-8274320-os-fork-and-exec-should-be-using-posix-spawn
 - JDK-8274320-os-fork-and-exec-should-be-using-posix-spawn

Changes: https://git.openjdk.java.net/jdk/pull/5698/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5698&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274320
  Stats: 43 lines in 4 files changed: 4 ins; 28 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5698.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5698/head:pull/5698

PR: https://git.openjdk.java.net/jdk/pull/5698