payload buffer datatypes are consistent

payload buffer datatypes are consistent

The 'data' parameter to nfq_get_payload() returns pointer to unsigned
char (rather than signed char) to make it consistent with the 'buf'
parameter of nfq_set_verdict(), nfq_set_verdict2(), and
nfq_set_verdict_mark(), all of which refer to the same data.  Either
signed or unsigned is fine, but they should be consistent as the output
of nfq_get_payload() may be passed back into nfq_set_verdict*(); in that
case, this change eliminates the need for typecasting in the calling
code when using compilers that enforce strict typecasting.

Signed-off-by: David Favro <netfilter@meta-dynamic.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>