affile: Treat /dev/kmsg on linux specially

On Linux, use a dgram reader for /dev/kmsg, because it works almost
exactly like a dgram source anyway, and a dgram reader is more
efficient than the indented-multiline one.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
system-source: Add Linux 3.5+ /dev/kmsg support

When on Linux, and /dev/kmsg is seekable, use that over /proc/kmsg,
using indented-multiline and the linux-kmsg format.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>