[Python 3] bytes-like strings decoding for saslauthd.py

Summary:
byte-like strings needed to be decoded as text strings

debugging with 2.7 origin code returned unicode strings, but returned in python 3 bytse-like string variables.

Optparse gets text strings from shell.
configparse uses internally text strings.
From version 3.0, python-ldap uses text where appropriate. On Python 2, the bytes mode setting influences how text is handled.

socket.streams and db including bytes-like string code, which will be decoded with this diff, for python-ldap operations.

Reviewers: PyKolab Developers, vanmeeuwen, sicherha

Reviewed By: PyKolab Developers, sicherha

Subscribers: sicherha, vanmeeuwen

Differential Revision: https://git.kolab.org/D3469