Exempt unspendable transaction outputs from dust checks

Since unspendable outputs can't be spent, there is no threshold at which it would be uneconomic to spend them.

This primarily targets transaction outputs with OP_RETURN.

Initially based on:

commit 9cf0ae26350033d43d5dd3c95054c0d1b1641eda
Author: zathras-crypto <zathrasc@gmail.com>
Date: Wed Mar 25 02:04:02 2015 -0700

Changes:

• cherry-picked on top of bitcoin:master
• added RPC test for fundrawtransaction