system-source: Make /proc/kmsg optional

In certain environments (vserver and OpenVZ come to mind), /proc/kmsg is
not readable, not even for root. On these systems, emit a warning, and
skip it, instead of aborting the startup.

Reported-by: Tamas Pal <folti@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>