Executive summary of the patch:

The attached patch removes the use of __private attributes in the smtpd
module allowing it to be extensible without needing to use the
"_<classname>__<attributename>" hack.


Summary of the patch's changes:

1. removes the unused __conn and __addr attributes
2. renames __server to smtp_server
3. renames __lines to received_lines
4. renames __state to smtp_state
5. renames __greeting to seen_greeting, and alters the default to empty
string to match the anticipated data
6. renames __mailfrom to mailfrom
7. renames __date to received_data
8. renames __fqdn to fqdn
9. removes __peer and uses base class' addr attribute

The existing unit tests contained within test_smtplib pass. Additional
tests could be written if it's deemed necessary.

There is a chance this patch will break backward compatibility with
programs that use the private-variable-access hack. A more complex patch
could be written providing greater compatibility if it's deemed necessary.