[client] input: add support for evdev keyboard capture mode

The new configuration option `input:evdev` accepts a comma separated
list of `/dev/input/` keyboard devices to use for input when in capture
mode. This makes it possible to capture only a specific keyboard instead
of all keyboards.

client/include/evdev.h

@@ -0,0 +1,46 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2025 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#include <stdbool.h>
+
+/**
+ * initialize configuration options
+ */
+void evdev_earlyInit(void);
+
+/**
+ * start the evdev layer
+ */
+bool evdev_start(void);
+
+/**
+ * stop the evdev layer
+ */
+void evdev_stop(void);
+
+/**
+ * grab the keyboard for exclusive access
+ */
+void evdev_grabKeyboard(void);
+
+/**
+ * ungrab the keyboard
+ */
+void evdev_ungrabKeyboard(void);