Process wait() improvements

[giampaolo]

• Process.wait() on POSIX now returns an enum showing the negative which was used to terminate the process:

>>> 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 psutil.Popen: inherit from subprocess + support wait(timeout=...) parameter #1736: psutil.Popen uses original subprocess.Popen.wait method (safer)