New cache manager add cache api

New cache manager add cache api

This patch is an attempt to add a new nl_cache_mngr_add_cache api
to allow adding an existing cache to cache manager.

Since the new api is similar to nl_cache_mngr_add
except for allocating the cache, the patch moves most of the
nl_cache_mngr_add code to nl_cache_mngr_add_cache and changes
nl_cache_mngr_add to call nl_cache_mngr_add_cache.

One use case for this api as pointed out by thomas would be to set cache
flags before associating the cache with a cache manager.

nl_cache_alloc_name("route/link", &cache);
nl_cache_set_flags(cache, NL_CACHE_AF_ITER);
nl_cache_mngr_add_cache(mngr, cache, ...);

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