[Patch] libnetclient private data fixes

[Albrecht Dreß <albrecht dress arcor de>]

Hi all,

related to Peter's work (commit 162b1faf1960fb7e0dee217e2e793505d1d7ba8a) I re-worked the other libnetclient 
classes to use the “standard” way for allocating private data.  Additionally, I updated the expired unit test 
certificates and an issue with coverage generation for newer lcov versions.

Cheers,
Albrecht.

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Patch details:
- libnetclient/net-client-pop.c, libnetclient/net-client-siobuf.c, libnetclient/net-client-smtp.c: properly 
access and destroy the private data
- libnetclient/test/Makefile.am: fix lcov cleaning the test app from the coverage data
- libnetclient/test/cert.pem, libnetclient/test/cert_u.pem: replace expired snakeoil certs for testing

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