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