xtables-restore: Unbreak *tables-restore

xtables-restore: Unbreak *tables-restore

Commit 3dc433b55bbfa ("xtables-restore: Fix --table parameter check")
installed an error check which evaluated true in all cases as all
callers of do_command callbacks pass a pointer to a table name already.
Attached test case passed as it tested error condition only.

Fix the whole mess by introducing a boolean to indicate whether a table
parameter was seen already. Extend the test case to cover positive as
well as negative behaviour and to test ebtables-restore and
ip6tables-restore as well. Also add the required checking code to the
latter since the original commit missed it.

Fixes: 3dc433b55bbfa ("xtables-restore: Fix --table parameter check")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>