build: check actually-used BSD link state fields

build: check actually-used BSD link state fields

ifi_link_state missing in OS X. There could be other *BSD's that haven't
implemented it and possibly affects older implementations.

The existing HAVE_BSD_LINK_DETECT configure.ac check is only confirming
the link state detection using ifmediareq.ifm_status found in
<net/if_media.h>. This is the link state detection used in
zebra/ioctl.c. Later, *BSD redefined struct if_data in <net/if.h> and
included link state detection. This is the method used in
zebra/kernel_socket.c

Additional test defined in config.ac to test for member struct
if_data.ifi_link_state defined in <net/if.h> separate from test for
<net/if_media.h> ifmediareq.ifm_status

Fixed #ifdef's in zebra/kernel_socket.c to use the new #define
No impact on older function calls in zebra/ioctl.c

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

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