The preexec_fn should be executed before closing the file descriptors.

[eldipa]

Currently, the preexec_fn is executed after the file descriptors are closed. This has some unwanted effects:

• if preexec_fn opens a file descriptor, it will be inherit by the child process
• if preexec_fn relays on having some file descriptor open, it will crash (see pexpect/pexpect#368)

The proposal is to move the "close the fds" section below the "execute the preexec_fn" code: https://github.com/pexpect/ptyprocess/blob/master/ptyprocess/ptyprocess.py#L266-L285

For reference, this is what it does subprocess.Popen: https://github.com/python/cpython/blob/master/Modules/_posixsubprocess.c#L528-L549

If it is okay, I can do a PR with the fix but I would like to hear your opinions about this.