src: get rid of printf

src: get rid of printf

This patch introduces nft_print()/nft_gmp_print() functions which have
to be used instead of printf to output information that were previously
send to stdout. These functions print to a FILE pointer defined in
struct output_ctx. It is set by calling:

| old_fp = nft_ctx_set_output(ctx, new_fp);

Having an application-defined FILE pointer is actually quite flexible:
Using fmemopen() or even fopencookie(), an application gains full
control over what is printed and where it should go to.

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