Make EOS Mirror Lock control work again

[anjok]

Is your feature request related to a problem? Please describe.
En-/Disabling Mirror Lock used to be supported, but is apparently commented out because it didn't work with some models.

Also the procedure used is probably wrong. I blindly tried the string in the code with my 450D and 6D (which you kids shouldn't do at home) and it set the camera in a weird state which was fixed by disconnecting and turning it on/off again. But that didn't alter the Mirror lock.

So after some further investigation and using the procedure outlined in:

https://github.com/gphoto/libgphoto2/blob/master/camlibs/ptp2/canon-eos-customfunc.txt

it turned out that the string for the 450D is different:

off: d4,4,1,20,2,101,1,0,10f,1,0,2,38,4,201,1,0,202,1,0,203,1,0,204,1,0,3,2c,3,50e,1,0,511,1,0,60f,1,0,4,38,4,701,1,0,704,1,0,811,1,0,80f,1,0,
on: d4,4,1,20,2,101,1,0,10f,1,0,2,38,4,201,1,0,202,1,0,203,1,0,204,1,0,3,2c,3,50e,1,0,511,1,0,60f,1,1,4,38,4,701,1,0,704,1,0,811,1,0,80f,1,0,

Note these actually toggle the on my 450D, no idea if it works for others.

And the 6D is different again. There's a property EOS_MirrorLockupState for which I have am Issue open and trying to figure out myself:

gphoto/libgphoto2#913

Describe the solution you'd like
Now, since the single fixed customfuncex string is apparently not working, it would be better to have either two strings for on/off which the user can set. This is for those cams that use customfuncex. Or put it in some config file that the user can change if he needs to.

For the 6D and others, I hope the property can be made writable and simple be toggled when mirror lock time is changed to <2 = > off and >=2 => on.

Describe alternatives you've considered
It would be nicer if there was a --set-config eos-mirror-lock=on in gphoto, but I'm even less knowledgable in that code base than in indi. And since it's not clear if the customfuncex value varies by model or even cam, I'm not sure that's managebable for them.

[anjok]

OK, so the dev version of gphoto now has a flag /main//status/mirrorup (name subject to change). While it would be nice if the indi driver could support the logic outlined above (<2s -> lock off, >=2s lock on), it's not really needed.