Werk #1571: check_mk_agent.linux: fix output of lnx_if on Ubuntu 8.04 (on older kernels), repairs tcp_conn_stats

Problem on some older kernels is, that /sys/class/net/$eth/address does not contain a newline (or does not exist at all). We now always output an additional newline. This does not harm and fixes the problem.

Without the fix the next section tcp_conn_stats is not correctly detected and is missing.

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