Merge #21250: build: make HAVE_O_CLOEXEC available outside LevelDB (bugfix)

9bac71350d98580cc7441957fc7c3fa2f4158553 build: make HAVE_O_CLOEXEC available outside LevelDB (bugfix) (Sebastian Falbesoner)
584fd91d2d294883e6896dbd64a2176528e94581 init: only use pipe2 if availabile, check in configure (Sebastian Falbesoner)

Pull request description:

  The result of the O_CLOEXEC availability check is currently only set in the Makefile and passed to LevelDB (see `LEVELDB_CPPFLAGS_INT` in `src/Makefile.leveldb.include`), but not defined to be used in our codebase. This means that code within the preprocessor conditional `#if HAVE_O_CLOEXEC` was actually never compiled. On the master branch this is currently used for pipe creation in `src/shutdown.cpp`, PR #21007 moves this part to a new module (I found the issue while testing that PR).

  The fix is similar to the one in #19803, which solved the same problem for HAVE_FDATASYNC.

  In the course of working on the PR it turned out that pipe2 is not available an all platforms, hence a configure check and a corresponding define HAVE_PIPE2 is introduced and used.

  The PR can be tested by anyone with a system that has pipe2 and O_CLOEXEC available by putting gibberish into the HAVE_O_CLOEXEC block: on master, everything should compile fine, on PR, the compiler should abort with an error. At least that's my naive way of testing preprocessor logic, happy to hear more sophisticated ways :-)

ACKs for top commit:
  laanwj:
    Code review ACK 9bac71350d98580cc7441957fc7c3fa2f4158553

Tree-SHA512: aec89faf6ba52b6f014c610ebef7b725d9e967207d58b42a4a71afc9f1268fcb673ecc85b33a2a3debba8105a304dd7edaba4208c5373fcef2ab83e48a170051

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