OpenBSD fixes

[giampaolo]

Fixes some Process methods which were broken on OpenBSD when the process is terminated (not getting NSP). Methods are: connections(), num_fds(), threads().
Also FreeBSD pids() implementation was refactored and cleaneup up.

======================================================================
ERROR: psutil.tests.test_misc.TestMisc.test_serialization
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/root/psutil/psutil/tests/test_misc.py", line 357, in test_serialization
    check(psutil.Process().as_dict())
  File "/root/psutil/psutil/__init__.py", line 538, in as_dict
    ret = meth()
  File "/root/psutil/psutil/__init__.py", line 886, in threads
    return self._proc.threads()
  File "/root/psutil/psutil/_psbsd.py", line 552, in wrapper
    return fun(self, *args, **kwargs)
  File "/root/psutil/psutil/_psbsd.py", line 750, in threads
    rawlist = cext.proc_threads(self.pid)
RuntimeError: kvm_openfiles() syscall failed

======================================================================
ERROR: psutil.tests.test_posix.TestProcess.test_num_fds
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/root/psutil/psutil/tests/test_posix.py", line 317, in test_num_fds
    call(p, name)
  File "/root/psutil/psutil/tests/test_posix.py", line 297, in call
    attr(*args)
  File "/root/psutil/psutil/__init__.py", line 876, in num_threads
    return self._proc.num_threads()
  File "/root/psutil/psutil/_psbsd.py", line 552, in wrapper
    return fun(self, *args, **kwargs)
  File "/root/psutil/psutil/_psbsd.py", line 738, in num_threads
    return len(self.threads())
  File "/root/psutil/psutil/_psbsd.py", line 552, in wrapper
    return fun(self, *args, **kwargs)
  File "/root/psutil/psutil/_psbsd.py", line 750, in threads
    rawlist = cext.proc_threads(self.pid)
RuntimeError: kvm_openfiles() syscall failed

======================================================================
ERROR: psutil.tests.test_process.TestProcess.test_halfway_terminated_process
----------------------------------------------------------------------
ProcessLookupError: [Errno 3] No such process

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/root/psutil/psutil/tests/test_process.py", line 1319, in test_halfway_terminated_process
    ret = meth()
  File "/root/psutil/psutil/__init__.py", line 1178, in connections
    return self._proc.connections(kind)
  File "/root/psutil/psutil/_psbsd.py", line 552, in wrapper
    return fun(self, *args, **kwargs)
  File "/root/psutil/psutil/_psbsd.py", line 780, in connections
    rawlist = cext.proc_connections(self.pid, families, types)
SystemError: <class 'OSError'> returned a result with an error set

======================================================================
FAIL: psutil.tests.test_contracts.TestFetchAllProcesses.test_fetch_all
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/root/psutil/psutil/tests/test_contracts.py", line 417, in test_fetch_all
    self.fail(''.join(failures))
AssertionError:     
    ======================================================================
    FAIL: test_num_threads (proc=psutil.Process(pid=0, name='swapper', started='21:26:30'))
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/root/psutil/psutil/tests/test_contracts.py", line 384, in test_fetch_all
        ret = self.call_meth(p, name)
      File "/root/psutil/psutil/tests/test_contracts.py", line 373, in call_meth
        return attr(*args, **kwargs)
      File "/root/psutil/psutil/__init__.py", line 876, in num_threads
        return self._proc.num_threads()
      File "/root/psutil/psutil/_psbsd.py", line 552, in wrapper
        return fun(self, *args, **kwargs)
      File "/root/psutil/psutil/_psbsd.py", line 738, in num_threads
        return len(self.threads())
      File "/root/psutil/psutil/_psbsd.py", line 552, in wrapper
        return fun(self, *args, **kwargs)
      File "/root/psutil/psutil/_psbsd.py", line 750, in threads
        rawlist = cext.proc_threads(self.pid)
    RuntimeError: kvm_openfiles() syscall failed