non-modified payload arguments are pointer-to-const

non-modified payload arguments are pointer-to-const

The payload parameters to nfq_set_verdict(), nfq_set_verdict2(), and
nfq_set_verdict_mark() are not modified by those functions, and
therefore should have datatype pointer-to-const.  This both causes the
source-code to more effectively represent what is the purpose of the
parameter, and eliminates the need to cast away const-ness when calling
the functions with compilers that enforce strict casting.  All existing
calling code should not need modification as pointer-to-X automatically
converts to pointer-to-const-X.

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