fix NULL server-identity TLS warning with recent gio
- libnetclient/net-client.c: use the remote host name as expected server identity for TLS connections. This fixes a strange warning when using recent GIO versions (see issue #34 (closed))
- libnetclient/test: replace the snakeoil test certificates as the old ones are not accepted any more by GnuTLS for security reasons; fix unit tests; add some more Valgrind suppressions for false-positives