Make exit code indicate a problem when pidfile is not writable.

Here, we try to open the pidfile for writing prior to forking so that
the exit code can properly indicate to the user that there was a
problem. No error messages are printed to the console in this case
because when --daemon implies --syslog. So, the syslog will contain
the message indicating that the pidfile wasn't writeable.

Fixes bug #598.

sshuttle/client.py

@@ -80,13 +80,25 @@ def check_daemon(pidfile):
 
 
 def daemonize():
+    # Try to open the pidfile prior to forking. If there is a problem,
+    # the client can then exit with a proper exit status code and
+    # message.
+    try:
+        outfd = os.open(_pidname, os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o666)
+    except PermissionError:
+        # User will have to look in syslog for error message since
+        # --daemon implies --syslog, all output gets redirected to
+        # syslog.
+        raise Fatal("failed to create/write pidfile %s" % _pidname)
+
+    # Create a daemon process with a new session id.
     if os.fork():
         os._exit(0)
     os.setsid()
     if os.fork():
         os._exit(0)
 
-    outfd = os.open(_pidname, os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o666)
+    # Write pid to the pidfile.
     try:
         os.write(outfd, b'%d\n' % os.getpid())
     finally: