RFC: Use pipe2 and O_CLOEXEC to check if exec succeeds after fork

The problem with issue #8002 and #6914 is that the ``exec()`` call fails after ``fork()``. After the failure, the parent process does not know about the failure. The parent process hangs during the ``waitpid()`` call.

I tried to use ``pipe2`` and ``O_CLOEXEC`` to check in the parent process if ``exec()`` succeeds after ``fork()``.

This PR fixes the problem with `pandoc'. I tested this on the latest master revision.

~~~shell
$ rustdoc --output-dir doc btree.rs
rust: task failed at 'failure in execvp: errno=No such file or directory', /home/smvv/work/rust/rust/src/libstd/run.rs:689
~~~

However, there is still some work to do before it can be merged:

 - Code has a hardcoded ``O_CLOEXEC`` value. This should be derived from the C header file. What would be the approach to derive this properly? ``O_CLOEXEC`` is defined in: http://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html
It is also possible to use ``fcntl()`` to set ``FD_CLOEXEC``, but the ``fcntl`` defined in libc.rs takes only two arguments instead of three.

 - Code uses ``pipe2``, which is not defined in libc.rs. Should ``pipe2`` be added to libc.rs, or what would be the way to proceed here?

 - There are some comments in the patch about ``false`` being used in the FILE reader and writer constructor. I was not sure if it should use ``false`` or ``true``.

I read that ``pipe2`` is Linux-specific, but perhaps it is also available on other Unix platforms?