kernel: Implement O_CLOEXEC

kernel: Implement O_CLOEXEC

* Using fcntl(2) just after open(2) is not enough to avoid race conditions
  in programs doing fork+exec sequences. Child processes may be created
  before fcntl() is run and inherit the parent's file descriptors.

* In some circonstances this behavior may even create security issues.

* O_CLOEXEC can be used to atomically set the close-on-exec flag for new
  file descriptors, avoiding the whole mess in the first place.

* Fixes issue #2356

Inspired-from: NetBSD