Execute targetcli commands concurrently raise EOFError about configshell-fb

[zoumingzhe]

I think save in class Prefs used the wrong lock type 'fcntl.LOCK_UN'.
Please see pull #43 for details.
Then, call open via 'wb' will truncate the file without lock protection,
if load prefs.bin at the same time, maybe read empty prefs.bin and
cause EOFError with very low probability.

    def save(self, filename=None):
        '''
        Saves the preferences to disk. If filename is not specified,
        use the default one if it is set, else do nothing.
        @param filename: Optional alternate file to use.
        @type filename: str
        '''
        if filename is None:
            filename = self.filename

        if filename is not None:
            fsock = open(filename, 'wb')
            fcntl.lockf(fsock, fcntl.LOCK_UN)
            try:
                six.moves.cPickle.dump(self._prefs, fsock, 2)
            finally:
                fsock.close()

    def load(self, filename=None):
        '''
        Loads the preferences from file. Use either the supplied filename,
        or the default one if set. Else, do nothing.
        '''
        if filename is None:
            filename = self.filename

        if filename is not None and os.path.exists(filename):
            fsock = open(filename, 'rb')
            fcntl.lockf(fsock, fcntl.LOCK_SH)
            try:
                self._prefs = six.moves.cPickle.load(fsock)
            finally:
                fsock.close()