Update smbserver.py and negtelnetserver.py to be compatible with
Python 3 while staying backwards-compatible to support Python 2.
Fix string encoding and handling of echoed and transferred data.
Tested with both Python 2.7.17 and Python 3.7.7
Reported-by: Daniel Stenberg
Assisted-by: Kamil Dudka
Reviewed-by: Marcel Raad
Fixes#5104Closes#5110