This commit starts to set the CLOEXEC flag for all files and sockets opened by
the standard library by default on all unix platforms. There are a few points of
note in this commit:

* The implementation is not 100% satisfactory in the face of threads. File
  descriptors only have the `F_CLOEXEC` flag set *after* they are opened,
  allowing for a fork/exec to happen in the middle and leak the descriptor.
  Some platforms do support atomically opening a descriptor while setting the
  `CLOEXEC` flag, and it is left as a future extension to bind these apis as it
  is unclear how to do so nicely at this time.

* The implementation does not offer a method of opting into the old behavior of
  not setting `CLOEXEC`. This will possibly be added in the future through
  extensions on `OpenOptions`, for example.

* This change does not yet audit any Windows APIs to see if the handles are
  inherited by default by accident.

This is a breaking change for users who call `fork` or `exec` outside of the
standard library itself and expect file descriptors to be inherted. All file
descriptors created by the standard library will no longer be inherited.

[breaking-change]