xtables-restore: Fix verbose mode table flushing

When called with --verbose mode, iptables-nft-restore did not print
anything when flushing the table. Fix this by adding a "manual" mode to
nft_cmd_table_flush(), turning it into a wrapper around '-F' and '-X'
commands, which is exactly what iptables-legacy-restore does to flush a
table. This though requires a real cache, so don't set NFT_CL_FAKE then.

Signed-off-by: Phil Sutter <phil@nwl.cc>