libcamera: media_device: Add functions to lock device for other processes

Add lock() and unlock() which are backed by lockf() and allow an
instance of libcamera to claim exclusive access to a media device.
These functions are the base of allowing multiple user of libcamera to
coexist in the system without stepping on each other's toes.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/include/media_device.h

@@ -30,6 +30,9 @@ public:
 	void release();
 	bool busy() const { return acquired_; }
 
+	bool lock();
+	void unlock();
+
 	int populate();
 	bool valid() const { return valid_; }
 
@@ -58,6 +61,7 @@ private:
 	int fd_;
 	bool valid_;
 	bool acquired_;
+	bool lockOwner_;
 
 	int open();
 	void close();