RFR: 8364312: debug agent should set FD_CLOEXEC flag rather than explicitly closing every open file

[Guanqiang Han]

Create a new function that marks all file descriptors found in /proc/self/fd with the FD_CLOEXEC flag to ensure they are automatically closed upon execution of a new program via exec().

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Commit messages:
 - 8364312: debug agent should set FD_CLOEXEC flag rather than explicitly closing every open file

Changes: https://git.openjdk.org/jdk/pull/26568/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26568&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364312
  Stats: 49 lines in 1 file changed: 19 ins; 18 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/26568.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26568/head:pull/26568

PR: https://git.openjdk.org/jdk/pull/26568