xtables-restore: Fix --table parameter check

xtables-restore: Fix --table parameter check

Xtables-restore tries to reject rule commands in input which contain a
--table parameter (since it is adding this itself based on the previous
table line). The manual check was not perfect though as it caught any
parameter starting with a dash and containing a 't' somewhere, even in
rule comments:

| *filter
| -A FORWARD -m comment --comment "- allow this one" -j ACCEPT
| COMMIT

Instead of error-prone manual checking, go a much simpler route: All
do_command callbacks are passed a boolean indicating they're called from
*tables-restore. React upon this when handling a table parameter and
error out if it's not the first one.

Fixes: f8e5ebc5986bf ("iptables: Fix crash on malformed iptables-restore")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Florian Westphal <fw@strlen.de>