agent/mibgroup/ip-mib/data_access/defaultrouter_linux: Rework netlink buffer alignment

Based on a patch from Joshua Hoke. See also
https://sourceforge.net/p/net-snmp/bugs/2547/.
Fix FreeBSD system define chain
Merge branch 'V5-4-patches' into V5-5-patches

* V5-4-patches:
  Fix FreeBSD system define chain