Fix unpack of abstract socket addrs with nul byte

Fix unpack of abstract socket addrs with nul byte

Addresses of Linux abstract namespace sockets are not nul-terminated C
strings, but rather an arbitrary character arrays. According to unix(7)
documentation from Linux, "Null bytes in the name have  no special
significance."

unpack_sockaddr_un() was just throwing the initial nul byte away and
then treating the rest like ordinary C string when computing the length
of the address, which was wrong. This fix utilizes the length of the PV
for addresses starting with nul instead.

The regression test was extended with check for the problem.