xtables-restore: Introduce rule counter tokenizer function

xtables-restore: Introduce rule counter tokenizer function

The same piece of code appears three times, introduce a function to take
care of tokenizing and error reporting.

Pass buffer pointer via reference so it can be updated to point to after
the counters (if found).

While being at it, drop pointless casting when passing pcnt/bcnt to
add_argv().

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