Multi-EKF support (ekf2)

- ekf2 can now run in multi-instance mode (currently up to 9 instances)
    - in multi mode all estimates are published to alternate topics (eg estimator_attitude instead of vehicle_attitude)
 - new ekf2 selector runs in multi-instance mode to monitor and compare all instances, selecting a primary (eg N x estimator_attitude => vehicle_attitude)
 - sensors module accel & gyro inconsistency checks are now relative to the mean of all instances, rather than the current primary (when active ekf2 selector is responsible for choosing primary accel & gyro)
 - existing consumers of estimator_status must check estimator_selector_status to select current primary instance status
 - ekf2 single instance mode is still fully supported and the default

Co-authored-by: Paul Riseborough <gncsolns@gmail.com>

src/lib/cdev/posix/cdev_platform.cpp

@@ -62,9 +62,9 @@ private:
 	px4_dev_t() = default;
 };
 
-static px4_dev_t *devmap[128] {};
+static px4_dev_t *devmap[256] {};
 
-#define PX4_MAX_FD 128
+#define PX4_MAX_FD 256
 static cdev::file_t filemap[PX4_MAX_FD] {};
 
 class VFile : public cdev::CDev