nfct: add network namespace support

nfct: add network namespace support

Allow the plugin to fetch data from a different network namespace. This
is possible by changing the network namespace before opening the netlink
socket, and immediately changing back to the original network namespace
once the socket is open. The number of nfct_open usages here warranted a
dedicated wrapper function.

If changing back to the original network namespace fails, ulogd will
log an error, but continue to run in a different network namespace than
it was started in, which may cause unexpected behaviour. But I don't see
a way to properly "escalate" it such that ulogd aborts entirely.

Also slightly adjust the error log messages to specify which socket
failed to open.

Signed-off-by: Corubba Smith <corubba@gmx.de>
Signed-off-by: Florian Westphal <fw@strlen.de>