hotplug.c: set nl_pid to zero

hotplug.c: set nl_pid to zero

With the current solution where nl_pid is set through getpid we run into
problems when running procd in a different PID namespace (e.g.
container). The PID number inside the active PID namespace will be set
which doesn't match the global PID. Therefore, procd will never receive
any netlink messages.

By setting nl_pid to zero the kernel will assign the global PID
automatically and fixes the issue.

Signed-off-by: Stefan Eichenberger <eichest@gmail.com>
Acked-by: John Crispin <john@phrozen.org>