#21725: Add RFC 6531 (SMTPUTF8) support to smtpd.

Patch by Milan Oberkirch, developed as part of his 2014 GSOC project.

Note that this also fixes a bug in mock_socket ('getpeername' was returning a
simple string instead of the tuple required for IPvX protocols), a bug in
DebugServer with respect to handling binary data (should have been fixed when
decode_data was introduced, but wasn't found until this patch was written),
and a long-standing bug in DebugServer (it was printing an extra blank line at
the end of the displayed message text).