tree-wide: use the return value from sockaddr_un_set_path()

tree-wide: use the return value from sockaddr_un_set_path()

It fully initializes the address structure, so no need for pre-initialization,
and also returns the length of the address, so no need to recalculate using
SOCKADDR_UN_LEN().

socklen_t is unsigned, so let's not use an int for it. (It doesn't matter, but
seems cleaner and more portable to not assume anything about the type.)