As reported by Alexander Hoogerhuis, the [DS]NAT targets do not allow use of
service names in the --to argument.  The same problem was fixed in the REDIRECT
target in commit 84d758b3 ("extensions: REDIRECT: fix --to-ports parser").
Use a similar fix here.

This closes bugzilla #514.

Phil

Signed-off-by: Phil Oester <kernel@linuxace.com>

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v2: also handle IPv6 [DS]NAT targets