Testnet/mainnet cross-reading of settings #759

In the settings, the Testnet seems to be reading from and setting some options for mainnet and not testnet. I have noticed this with at least the strDataDir option.

Replication steps on Ubuntu 15.10

1. Tail ~/.config/Bitcoin-Qt.conf and ~/.config/Bitcoin-Qt-testnet.conf

2. Reset the testnet settings by going to Settings > Reset Options. Notice how the testnet conf file changes.

3. Now start Bitcoin core with `-choosedatadir -testnet`.

After the third step, you will see that the strDataDir setting is not added to the testnet conf but is added/changed in the normal conf file.

Originally https://github.com/bitcoin/bitcoin/issues/8106