Initial netplay settings. The code is already there and seems to work
well. Just needed a way to change the settings to be able to use it.

Please note that at the moment, you have to change the netplay settings,
then "resume content" and enable netplay in the rgui menu.
Merge remote-tracking branch 'origin/master' into netplay
(Settings data) Implement read_handler for settings data
(Settings data) Building helper functions - make them public again
(iOS) Build fix
Merge remote-tracking branch 'RetroArch/master' into netplay_nfc