[Issue 21755] New: Add CLOEXEC posix flags declarations

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21755

          Issue ID: 21755
           Summary: Add CLOEXEC posix flags declarations
           Product: D
           Version: D2
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: dfj1esp02 at sneakemail.com

O_CLOEXEC, F_DUPFD_CLOEXEC, SOCK_CLOEXEC and SOCK_NONBLOCK declarations are
missing for linux. Linux also needs accept4 function, because CLOEXEC flag is
not inherited automatically.

O_CLOEXEC, SOCK_CLOEXEC and SOCK_NONBLOCK for NetBSD.

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