terminates the string in all cases, based on code from netstat(1).
The path in a sockaddr_un is terminated either by a '\0', or by
the end of the sockaddr as defined by sun_len.
Previously, the code could write the "safety" '\0' beyond the end
of the sockaddr (sockaddr_un's need only be large enough to store
sun_len bytes), and writing into the the supplied sockaddr is bad
anyway.