Process.exe() on FreeBSD may raise OSError(ENOENT)

[giampaolo]

======================================================================
FAIL: test_fetch_all (psutil.tests.test_process.TestFetchAllProcesses)
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Traceback (most recent call last):
  File "psutil/tests/test_process.py", line 1605, in test_fetch_all
    self.fail(''.join(failures))
AssertionError:     
    ======================================================================
    FAIL: test_exe (proc=psutil.Process(pid=4018, name='screen'))
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "psutil/tests/test_process.py", line 1570, in test_fetch_all
        ret = attr(*args)
      File "psutil/__init__.py", line 605, in exe
        exe = self._proc.exe()
      File "psutil/_psbsd.py", line 413, in wrapper
        return fun(self, *args, **kwargs)
      File "psutil/_psbsd.py", line 461, in exe
        return cext.proc_exe(self.pid)
    OSError: [Errno 2] No such file or directory

The error originates from sysctl used with KERN_PROC_PATHNAME. Also procstat fails, although with a different error:

user@freebsd~/psutil$ sudo procstat -b 4108
procstat: sysctl(kern.proc): No such process
procstat: procstat_getprocs()

What we can do is return an empty string instead, since this is already allowed by exe() API.