proton: Add waitforexit verb

The Steam client only sets critical environment variables for the game
process itself, not for getcompatpath etc. This verb lets the Steam
client wait for the prefix to shut down so it can restart the prefix
with the variables set.

Required to avoid duplicate controllers when controller config support
is enabled, for example.

proton

@@ -466,6 +466,9 @@ elif sys.argv[1] == "getnativepath":
     #windows -> linux path
     path = subprocess.check_output([wine_path, "winepath", sys.argv[2]], env=env, stderr=lfile)
     sys.stdout.write(path)
+elif sys.argv[1] == "waitforexit":
+    #wait for wineserver to shut down
+    run_wine([bindir + "/wineserver", "-w"])
 else:
     log("Need a verb.")
     sys.exit(1)