address: return NULL instead to fail in assert
netlink: trace address parsing as event in debug3
netlink: fixed tunnel ipv6 link address parsing
netlink: parse link hwpeer alias hwbrd address

The content depends on the IFF_POINTOPOINT flag; when set
is has to be interpreted as peer otherwise as brd. We use
a single hwpeer variable in the link struct for both.
Merge pull request #191 from mtomaschewski/address-parsing

netlink address parsing fixes