Input: Some more followups to the input device auto-detection stuff (#11855)

Switch to a new `input.fdopen` API & wrapper so we can keep the fds opened by `fbink_input_scan` instead of closing them to re-open them right after that...

This should hopefully help on racy zForce devices that attempt to handle power management when opening/closing the device. We know this sometimes horribly fail to re-activate the IR grid (c.f., our manual activation on resume), but this apparently could also happen here (re: #11844) because of the quick succession of open->close->open.

frontend/device/input.lua

@@ -323,6 +323,29 @@ function Input.open(path, name)
     end
 end
 
+--[[--
+Wrapper for our Lua/C input module's fdopen.
+
+Note that we adhere to the "." syntax here for compatibility.
+
+The `name` argument is optional, and used for logging purposes only.
+`path` is mandatory, though!
+--]]
+function Input.fdopen(fd, path, name)
+    -- Make sure we don't open the same device twice.
+    if not Input.opened_devices[path] then
+        input.fdopen(fd)
+        -- As with input.open, it will throw on error (closing the fd first)
+        Input.opened_devices[path] = fd
+        if name then
+            logger.dbg("Kept fd", fd, "open for input device", name, "@", path)
+        else
+            logger.dbg("Kept fd", fd, "open for input device @", path)
+        end
+        return fd
+    end
+end
+
 --[[--
 Wrapper for our Lua/C input module's close.