Fixes#1563
This implements dmesg reading via `/dev/kmsg`, with message parsing and
formatting. Kernel log facility and severity are parsed, timestamp is
calculated relative to boot time (it's accurate unless time jumps a
lot during node lifetime).
New flags to follow dmesg was added, tail flag allows to stream only new
message (ignoring old messages). We could try to implement tailing last
N messages, just a bit more work, open to suggestions (for symmetry with
regular logs).
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
