src/libs/zbxsysinfo/netbsd/net.c: convert to using the now-preferred sysctl() method.

[he32]

The sysctl() method of getting at the network statistics is the preferred method on NetBSD nowadays. The code to do so is lifted from NetBSD's usr.bin/netstat/if.c and adapted as required.

This fixes a build problem on netbsd-10 where the internal data structure has changed to use per-CPU counters, and the code has been tested on NetBSD 8.2, so this method also works in now quite old NetBSD hosts.