Update libretro.cpp

Try to map mednafen input devices as core options
added core settings for analog type dualshock|dualanalog|analogjoy
changed analog core settings so they affect both ports
- fixed previous commit so it doesn't modify anything on settings.cpp nor settings.h
- now you can change controllers on the fly
Merge pull request #33 from fr500/master

Mapped dualanalog and flightstick modes to core setting