Add NL_CACHE_AF_ITER support during refill and resync

Add NL_CACHE_AF_ITER support during refill and resync

This patch adds support to iter over all supported families
during cache fill and resync.

The motivation for this was previously introduced at
http://lists.infradead.org/pipermail/libnl/2012-November/000734.html
In short, this patch allows caches to request dump on all supported
families instead of only AF_UNSPEC as done today.

With feedback from thomas this patch makes the iter over all families
conditional on per cache flag NL_CACHE_AF_ITER

Signed-off-by: Thomas Graf <tgraf@suug.ch>