filter: PWSNIFF: replace malloc+strncpy with strndup

filter: PWSNIFF: replace malloc+strncpy with strndup

There are a couple of instances of allocating memory with `malloc`,
followed by copying a string to it with `strncpy` and adding an explicit
assignment of `\0` to terminate the string.  Replace them with
`strndup`.

Add an enum to name indices of output keys.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>