RFR: 8357089: Remove VFORK launch mechanism from Process implementation (linux)

[Thomas Stuefe]

See  the companion CSR (https://bugs.openjdk.org/browse/JDK-8357090) for the ratio behind this removal.

Patch

- removes all code handling the VFORK mode.
- removes or rewrites comments explaining use of vfork by the JVM.
- we now print out an error message to stderr if the user still specifies `-Djdk.lang.Process.launchMechanism=vfork`.

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Commit messages:
 - add warning message if user specifies VFORK
 - start

Changes: https://git.openjdk.org/jdk/pull/25260/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25260&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357089
  Stats: 106 lines in 4 files changed: 0 ins; 84 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/25260.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25260/head:pull/25260

PR: https://git.openjdk.org/jdk/pull/25260