Set netlink socket to be non-blocking

Set netlink socket to be non-blocking

Even though we use select/poll to explicitly query when the netlink
socket is ready for read, sometimes we end up reading from the socket
when it is not ready to read and then the process hangs for several
seconds (20-30s). Avoid this situation by setting the socket to be
non-blocking, so we get a status in this case that allows us to continue.

Change-Id: I35447c23a9350176007df5455bf9451021e9856d
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20230308151945.3670151-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26353.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 7aa3520768a68fb6a73ab64569c7be5d571f86fc)