src: add input flag NFT_CTX_INPUT_NO_DNS to avoid blocking

src: add input flag NFT_CTX_INPUT_NO_DNS to avoid blocking

getaddrinfo() blocks while trying to resolve the name. Blocking the
caller of the library is in many cases undesirable. Also, while
reconfiguring the firewall, it's not clear that resolving names via
the network will work or makes sense.

Add a new input flag NFT_CTX_INPUT_NO_DNS to opt-out from getaddrinfo()
and only accept plain IP addresses.

We could also use AI_NUMERICHOST with getaddrinfo() instead of
inet_pton(). By parsing via inet_pton(), we are better aware of
what we expect and can generate a better error message in case of
failure.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Reviewed-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>