prosodyctl: Abort and warn if the config can't be opened after dropping root privileges (fixes #990)

prosodyctl

@@ -164,6 +164,17 @@ if have_pposix and pposix then
 		if not switched_user then
 			-- Boo!
 			print("Warning: Couldn't switch to Prosody user/group '"..tostring(desired_user).."'/'"..tostring(desired_group).."': "..tostring(err));
+		else
+			-- Make sure the Prosody user can read the config
+			local conf, err, errno = io.open(ENV_CONFIG);
+			if conf then
+				conf:close();
+			else
+				print("The config file is not readable by the '"..desired_user.."' user.");
+				print("Prosody will not be able to read it.");
+				print("Error was "..err);
+				os.exit(1);
+			end
 		end
 	end