extensions: libxt_NFLOG: nflog-range does not truncate packets

extensions: libxt_NFLOG: nflog-range does not truncate packets

The option --nflog-range has never worked, but we cannot just fix this
because users might be using this feature option and their behavior would
change. Instead add a new option --nflog-size. This option works the same
way nflog-range should have, and both of them are mutually exclusive. When
someone uses --nflog-range we print a warning message informing them that
this feature has no effect.

To indicate the kernel that the user has set --nflog-size we have to pass a
new flag XT_NFLOG_F_COPY_LEN.

Also updated the man page to reflect the new option and added tests to
extensions/libxt_NFLOG.t

Reported-by: Joe Dollard <jdollard@akamai.com>
Reviewed-by: Josh Hunt <johunt@akamai.com>
Signed-off-by: Vishwanath Pai <vpai@akamai.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>