gevent / gevent / 2792

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Use /dev/fd|/proc/self/fd to get open FDs to close in Popen

If those aren't available, use the old brute-force approach. This is
closer to what CPython does in its C implementation, and is much
faster.

We don't have to worry about the async signal safe stuff the C code
does because, guess what, we're running Python code here already
anyway, so much of it could wind up doing something that's not
actually safe anyway. Oh well.

Since we depend on Python 3.4 and above now, we can rely on the
CLOEXEC flag being set by default and not have to manually check
everything.

This speeds up 2.7 (close_fds defaults to *false* there, so the
default case doesn't change):

| Benchmark                 | 27_bench_subprocess | 27_bench_subprocess_dirfd     |
+---------------------------+---------------------+-------------------------------+
| spawn native no close_fds | 1.81 ms             | 1.79 ms: 1.01x faster (-1%)   |
| spawn gevent no close_fds | 2.11 ms             | 2.20 ms: 1.04x slower (+4%)   |
| spawn native close_fds    | 31.0 ms             | 30.2 ms: 1.03x faster (-3%)   |
| spawn gevent close_fds    | 31.6 ms             | 2.56 ms: 12.31x faster (-92%) |

And it really speeds up 3.7 (close_fds defaults to *true* there, so
the default case is much faster, and the non-default case is even better):

| Benchmark                 | 37_bench_subprocess | 37_bench_subprocess_dirfd     |
+---------------------------+---------------------+-------------------------------+
| spawn native no close_fds | 1.34 ms             | 1.27 ms: 1.06x faster (-6%)   |
| spawn gevent no close_fds | 117 ms              | 3.05 ms: 38.27x faster (-97%) |
| spawn native close_fds    | 1.36 ms             | 1.30 ms: 1.04x faster (-4%)   |
| spawn gevent close_fds    | 32.5 ms             | 3.34 ms: 9.75x faster (-90%)  |

Fixes #1172

43 of 43 new or added lines in 1 file covered. (100.0%)

9839 of 11642 relevant lines covered (84.51%)

1.98 hits per line