afsocket: fix segmentation fault in the notls build

When SSL is disabled, the construct_transport() virtual method
was incorrectly initialized, causing an endless recursion during
initialization of any AF_INET destination. This in turn causes a stack
exhaustion, which in turn causes a segmentation fault.

Thanks to Howard for the detailed bug report and proposed fix.

Reported-by: Howard Tsai <htsai@skytap.com>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
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>