Ignore pipes for which the protocol is not set yet (still equal to None).

Set the _returncode attribute, so close() doesn't try to terminate the process.

subprocess, kill the subprocess (close pipes, kill and read the return status).
Log an error in such case.

Martin Richard.

If the process exited before the _post_init() method was called, scheduling the
call to _call_connection_lost() with call_soon() is wrong: connection_made()
must be called before connection_lost().

Reuse the BaseSubprocessTransport._call() method to schedule the call to
_call_connection_lost() to ensure that connection_made() and connection_lost()
are called in the correct order.

- Log stdin, stdout and stderr transports and protocols
- Log process identifier (pid)
- Log connection of pipes
- Log process exit
- Log Process.communicate() tasks: feed stdin, read stdout and stderr
- Add __repr__() method to many classes related to subprocesses

Store the pid so it is still accessible after the process exited. It's more
convinient for debug.

* Add a new asyncio.subprocess module
* Add new create_subprocess_exec() and create_subprocess_shell() functions
* The new asyncio.subprocess.SubprocessStreamProtocol creates stream readers
  for stdout and stderr and a stream writer for stdin.
* The new asyncio.subprocess.Process class offers an API close to the
  subprocess.Popen class:

  - pid, returncode, stdin, stdout and stderr attributes
  - communicate(), wait(), send_signal(), terminate() and kill() methods

* Remove STDIN (0), STDOUT (1) and STDERR (2) constants from base_subprocess
  and unix_events, to not be confused with the symbols with the same name of
  subprocess and asyncio.subprocess modules
* _ProactorBasePipeTransport.get_write_buffer_size() now counts also the size
  of the pending write
* _ProactorBaseWritePipeTransport._loop_writing() may now pause the protocol if
  the write buffer size is greater than the high water mark (64 KB by default)
* Add new subprocess examples: shell.py, subprocess_shell.py,
* subprocess_attach_read_pipe.py and subprocess_attach_write_pipe.py