zebra: fix struct msghdr initializers

zebra: fix struct msghdr initializers

struct msghdr field orders are not strictly specified in POSIX.
Improve portability by using designated initializer. This fixes
build against musl c-library where struct msghdr is POSIX
compliant (Linux kernel and glibc definitions are non-conforming).

As the result is also more readable, struct iovec initilizers
were also converted.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>