psutil only returns uids of user-owned process on Solaris 10

[gsauthof]

How to reproduce:

As normal user, chose a PID of another user:

>>> p = psutil.Process(12345)
>>> p.uids()
Traceback (most recent call last):
  File "/path/to/venv/lib/python3.5/site-packages/psutil/_pssunos.py", line 346, in wrapper
    return fun(self, *args, **kwargs)
  File "/path/to/venv/lib/python3.5/site-packages/psutil/_pssunos.py", line 477, in uids
    real, effective, saved, _, _, _ = self._proc_cred()
  File "/path/to/venv/lib/python3.5/site-packages/psutil/_common.py", line 336, in wrapper
    return fun(self)
  File "/path/to/venv/lib/python3.5/site-packages/psutil/_pssunos.py", line 400, in _proc_cred
    return cext.proc_cred(self.pid, self._procfs_path)
PermissionError: [Errno 13] Permission denied: '/proc/12345/cred'
 
During handling of the above exception, another exception occurred:
 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/to/venv/lib/python3.5/site-packages/psutil/_common.py", line 336, in wrapper
    return fun(self)
  File "/path/to/venv/lib/python3.5/site-packages/psutil/__init__.py", line 768, in uids
    return self._proc.uids()
  File "/path/to/venv/lib/python3.5/site-packages/psutil/_pssunos.py", line 362, in wrapper
    raise AccessDenied(self.pid, self._name)
psutil.AccessDenied: psutil.AccessDenied (pid=12345)

The thing is that psutil tries to read the uids from /proc/$pid/cred that is always only user-readable on Solaris 10:

$ ls /proc/12345/cred -l
-r-------- 1 juser juser 52 2017-11-23 11:27 /proc/12345/cred

A better choice on Solaris 10 is to use /proc/$pid/psinfo since it also contains the uids (modulo the saved user id) - and is readable by everyone. Thus, if psutil really cares about the saved user id, it could also try /proc/$pid/cred first and then fall back to /proc/$pid/psinfo.

For our example:

$ ls /proc/12345/psinfo -l
-r--r--r-- 1 juser juser 336 2017-11-23 11:27 /proc/12345/psinfo

[giampaolo]

Can you provide a PR?