Fixed handling of non-null-terminated unix sockets. At least FreeBSD, macOS, NetBSD, and OpenBSD can return unix sockets with non-null-terminated sun_path. Additionally, the address may become non-null-terminated if it does not fit into the buffer provided and was truncated (may happen on macOS, NetBSD, and Solaris, which allow unix socket addresess larger than struct sockaddr_un). As such, ngx_sock_ntop() might overread the sockaddr provided, as it used "%s" format and thus assumed null-terminated string. To fix this, the ngx_strnlen() function was introduced, and it is now used to calculate correct length of sun_path.