Process wait() improvements (#1747)

* `Process.wait()` on POSIX now returns an `enum` showing the negative which was used to terminate the process:
```python
>>> import psutil
>>> p = psutil.Process(9891)
>>> p.terminate()
>>> p.wait()
<Negsignal.SIGTERM: -15>
```

* the return value is cached so that the exit code can be retrieved on then next call, mimicking `subprocess.Popen.wait()`
* `Process` object provides more `status` and `exitcode` additional info on `str()` and `repr()`:

```
 >>> proc
 psutil.Process(pid=12739, name='python3', status='terminated', exitcode=<Negsigs.SIGTERM: -15>, started='15:08:20')
```

Extra:
* improved `wait()` doc
* reverted #1736: `psutil.Popen` uses original `subprocess.Popen.wait` method (safer)

1 files changed, 1 insertions, 1 deletions

diff --git a/README.rst b/README.rst
index 62ab3f23..2137a1a2 100644
--- a/README.rst
+++ b/README.rst
@@ -438,7 +438,7 @@ Process management
     >>> p.terminate()
     >>> p.kill()
     >>> p.wait(timeout=3)
-    0
+    <Exitcode.EX_OK: 0>
     >>>
     >>> psutil.test()
     USER         PID %CPU %MEM     VSZ     RSS TTY        START    TIME  COMMAND