system-source: Stricter /dev/kmsg requirements on Linux

On Linux 3.2, /dev/kmsg is present, is seekable, is pollable with
poll(), but not with epoll(). This leads to a run-time abort, because
syslog-ng can't monitor the file in this case. To fix the situation,
tighten the requirements in system-source, and check not only for
seekability, but pollability too.

Reported-by: Martin Bagge / brother <brother@bsnet.se>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>