Automatic backend selection in nsfb

I have previously patched netsurf and a couple of test programs to automatically select the appropriate nsfb backend using getenv() if it is not provided as an arg, but it may be useful to have this as the default behavior. the following env variables are set: Wayland -> WAYLAND_DISPLAY xcb -> DISPLAY linux framebuffer -> FRAMEBUFFER (and possibly isatty()?) vnc -> check port 5900? able -> no idea For the ones I'm familiar with the order should be wayland,xcb,linuxfb,sdl but I am less familiar with able and vnc. I assume Able should fall somewhere around the linuxfb and vnc should either be first (by checking port 5900) or required as an arg.