iptables: add optional [seconds] argument to -w

This patch adds an optional numeric argument
to -w option (added with 93587a0) so one can
specify how long to wait for an exclusive lock.

If the value isn't specified it works as before,
i.e. program waits indefinitely.

If user specifies it, program exits after
the given time interval passes.

This patch also adds the -w/--wait to nftables
compat code, so the parser doesn't complain.

[ In the original patch, iptables-compat -w X was not working,
  I have fixed by adding the dummy code not to break scripts
  using the new optional argument --pablo ]

Signed-off-by: Jiri Popelka <jpopelka@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>