[Fix] Fix copying of sockaddr_un addresses

[Fix] Fix copying of sockaddr_un addresses

They can be very tiny (hence the adjustment of the size assert)
and the path can contain intermittent null bytes, so the only choice
is to trust the input slen and copy the whole struct.

An autobound unix socket uses an abstract address, which starts with a
null byte, hence this change is neccesary for such an address getting
copied properly.