fixing cmdline() for empty-string argumemts on posix systems

[arnimarj]

No description provided.

[giampaolo]

Mmmm for extra safety I would do this instead:

    @wrap_exceptions
    def cmdline(self):
        fname = "/proc/%s/cmdline" % self.pid
        kw = dict(encoding=DEFAULT_ENCODING) if PY3 else dict()
        with open(fname, "rt", **kw) as f:
            data = f.read()
        if data.endswith('\x00'):  # this should always be true
            data = data[:-1]
        return [x for x in data.split('\x00') if x]

Please change this, and also:

• make test_psutil.py PRP8 compliant (new line are longer than 80 chars)
• update HISTORY.rst
• update CREDITS
  ...then it's ready to be merged.

[giampaolo]

@arnimarj : it turns out this caused #639. I should have fixed it in 3a620d9. Can you please confirm this works as expected?