RFR: 8364312: debug agent should set FD_CLOEXEC flag rather than explicitly closing every open file [v4]

[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().

Guanqiang Han has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - update some comments
 - Merge remote-tracking branch 'upstream/master' into 8364312
 - a small fix
 - Merge remote-tracking branch 'upstream/master' into 8364312
 - Update exec_md.c
   
   correct an compilation error
 - 8364312: debug agent should set FD_CLOEXEC flag rather than explicitly closing every open file
   
   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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Changes:
  - all: https://git.openjdk.org/jdk/pull/26568/files
  - new: https://git.openjdk.org/jdk/pull/26568/files/cd14e763..43363d34

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26568&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26568&range=02-03

  Stats: 601 lines in 38 files changed: 356 ins; 117 del; 128 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