libnftables: Introduce nft_ctx_flush_cache()

libnftables: Introduce nft_ctx_flush_cache()

This allows an application to explicitly flush caches associated with a
given nft context, as seen in cli_complete().

Note that this is a bit inconsistent in that it releases the global
interface cache, but nft_ctx_free() does the same so at least it's not a
regression.

Note that there is no need for explicit cache update routine since cache
is populated during command execution depending on whether it is needed
or not.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>