Support the SetModeDialDisable opcode for Canon cameras

[jensanjo]

Several Canon EOS cameras allow software setting of the exposure mode dial. This is supported by libgphoto2 via the autoexposuremodedial config.
This works as expected wit for instance the EOS 1200D and the EOS 2000D.
In some Canon cameras, for instance the Canon EOS R50, you need to unlock this feature before it can be used.

The SetModeDialDisable opcode (0x9088) has to be sent to the camera with parameter 0 to disable the mode dial and allow software setting of the exposure mode.
The disabling is cancelled by sending the SetModeDialDisable command with parameter 1.

This pull request will allow software setting of the exposure mode during a camera session by issuing the SetModeDialDisable as described above, by enabling software setting in camera_init and disabling it in camera_exit.

It also adds a disablemodedial camera action config so that the feature can be enabled / disabled during a camera session.

Being able to control the exposure mode via software is a valuable feature for many applications.

This pull request consists of 3 commits:

• b83256c adds the PTP_OC_CANON_SetModeDialDisable opcode in ptp.h and ptp.c
• ca24153 adds the enabling/disabling to camera_init and camera_exit for supported cameras
• 26f4e49 adds disablemodedial command to camera actions for supported cameras