improve STDOUT/STDERR logging redirection.

Instead of faking STDIN/STDERR to a fake object that we close on reopen,
simply redirect them to a file and reopen this one when need.

Should fix most of the issues on #309
add GUNICORN_INHERIT_FDS environment variable support

You can now pass a list of file descriptors to Gunicorn that won't be
closed when it starts. Allows someone to pass any stream to gunicorn or
set some redirection.
don't close on exec STDOUT/STDERR