d/vive: Tidy IMU, fix IMU, tidy config reading and switch to cJSON

This is a combination of @ryan.pavlik's !185 (closed) MR and my tidy branch.

Spotted something wrong with how the IMU code applied the gyro bias, assumed that also applied to the accelerometer.

Also makes cJSON files be accessible from everywhere in Monado, and adds a nice place to add common helpers.

Tested on Valve Index.