rule: allow src/dstnat prios in input and output

rule: allow src/dstnat prios in input and output

Dan Winship says:

The "dnat" command is usable from either "prerouting" or "output", but the
"dstnat" priority is only usable from "prerouting". (Likewise, "snat" is usable
from either "postrouting" or "input", but "srcnat" is only usable from
"postrouting".)

No need to restrict those priorities to pre/postrouting.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1694
Signed-off-by: Florian Westphal <fw@strlen.de>