We are now careful to not attach any Handle finalizers when creating
pipes for inter-process communication on Unix systems. Instead, the
handles are closed manually.

The finalizers were causing problems in situations such as the following:

  - the parent creates a new pipe, e.g. pipe2([7,8]),
  - the parent spawns a child process, and lets FD 8 be inherited by the child,
  - the parent closes FD 8 (as it should),
  - the parent opens FD 8 for some other purpose, e.g. for writing to a file,
  - the finalizer for the Handle wrapping FD 8 runs, closing FD 8, even though
    it is now in use for a completely different purpose.

This commit does not include a test, as the above bug is a bit difficult
to trigger.