Closes #1538. Refactors 'onionservicesonly' to 'onlynet=onion', prevents bootstrapping from non-onion nodes when true, and prevents bootstrapping from any node but the trusted peer (when set)

[sgj3]

This PR fixes #1538. It:

• Gracefully changes the config option 'onionservicesonly=true' to 'onlynet=onion', upgrading old config directives upon GUI settings dialog "OK"
• When onion services only is requested, prevents bootstrapping from a non-onion node
• When a trusted peer is set, prevents bootstrapping from any other node

This PR does not add any warning or error dialogs, which would be useful for a user unknowingly unable to bootstrap because they set 'onlynet=onion'.

(Please ignore #1693. I accidentally hit ENTER when typing the title, which GitHub oh-so-helpfully interpreted as a click on "Create pull request".)

[PeterSurda]

I need more time to look at it more closely, but upon first glance it looks ok, just fix the code quality issues and keep it rebased so that I can merge it when ready.