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

[Thomas Stuefe]

(for JDK 26)

Note: This PR is a continuation of the old PR here: https://github.com/openjdk/jdk/pull/25260; had to close the old one since I had Skara problems after the JDK25 split-off.

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 clarifies 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.

-------------

Commit messages:
 - warning message
 - remove-vfork

Changes: https://git.openjdk.org/jdk/pull/25768/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25768&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357089
  Stats: 122 lines in 4 files changed: 1 ins; 93 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/25768.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25768/head:pull/25768

PR: https://git.openjdk.org/jdk/pull/25768