FIFO reading: Better compatibility with various Unicode bytes

Some SIP headers such as "User-Agent" may contain Unicode data that
would crash the CLI on some OS/Python-3 combinations.  For example, an
"ul_dump" MI command may run into the following trace:

  File "/usr/lib/python3/dist-packages/opensipscli/communication/fifo.py", line 77, in execute
    replycmd = reply_fifo.readline()
  File "/usr/lib/python3.7/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf2 in position 2737: invalid continuation byte

This patch simply changes the default handling of byte-scanning errors,
by simply replacing unrecognized characters with a "?" symbol and moving
forward.

opensipscli/communication/fifo.py

@@ -73,7 +73,7 @@ def execute(method, params):
                 format(fifo_file, ex))
 
     logger.debug("reply file '{}'".format(reply_fifo_file))
-    with open(reply_fifo_file, 'r') as reply_fifo:
+    with open(reply_fifo_file, 'r', errors='replace') as reply_fifo:
         replycmd = reply_fifo.readline()
         #logger.debug("received reply '{}'".format(replycmd))