libnftables: Ensure output_fp is never NULL

libnftables: Ensure output_fp is never NULL

Initialize output_fp to 'stdout' upon context creation and check output
stream validity in nft_ctx_set_output(). This allows to drop checks in
nft_{gmp_,}print() and do_command_export(). While doing so for the
latter, simplify it a bit by using nft_print() which takes care of
flushing the output stream.

If applications desire to drop all output, they are supposed to open
/dev/null and assign that.

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