zebra: kernel_socket: fix 64bit MacOS X alignment

zebra: kernel_socket: fix 64bit MacOS X alignment

In OS X 10.7 zebra crashed on invalid execution address.

sockaddr padding in *_msghdr is observed to be 4 bytes in 64bit OS X.

The ROUNDUP macro assumed alignment on sizeof(long) which
allocates 8 bytes on 64bit systems, 4 bytes on 32bit systems
which is true for BSD generally.

Test for Apple and use sizeof(int) which allocates 4 bytes on 32 & 64bit
systems.

Tested on 64bit OS X 10.7, FreeBSD 9.0 amd64 & i386 (32bit)
using gcc & clang

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>