Fix rt_msghdr padding on NetBSD.

NetBSD uses a fixed 64-bit padding, unlike OS X (32-bit), and unlike
other platforms (sizeof(unsigned long)). There is an RT_ROUNDUP macro
that does the right alignment; use that when available.

http://ftp.netbsd.org/pub/NetBSD/NetBSD-current/src/lib/libc/net/getifaddrs.c
http://fxr.watson.org/fxr/source/net/route.h?v=NETBSD#L270

Rounding to 4 bytes was breaking with IPv6 addresses, which are 28
bytes. The pointer was being advanced by 28 instead of 32. Compare to
r29739, somewhat similar.

Reported by Fredrik Pettai.
http://seclists.org/nmap-dev/2013/q1/384