[Debian-on-mobile-maintainers] Bug#1051465: unl0kr: Lacks automated migration when osk-sdl is already installed

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Thanks for getting the ball rolling on this one.

I think in the first instance we should switch c-s-m to unl0kr to catch 
new installs as you say. That'll stop the problem from getting worse. It 
would probably be a good idea to ask more technical users to make the 
switch too before making this type of change.

After that, I have a couple of thoughts on the automated transition:

1. If unl0kr is installed while osk-sdl is it should probably do 
nothing. This avoids breaking working installs.

2. If unl0kr is installed and osk-sdl isn't it should check for 
osk-sdl's debconf setting indicating that c-s-m or similar configured 
crypttab in the first place. If this is set unl0kr could attempt to add 
its keyscript to the crypttab.

     a. This probably also requires a release of osk-sdl with the 
inverse to:

         * Deconfigure itself

         * Configure unl0kr

         * Set unl0kr's debconf flag as osk-sdl's is.

3. A new install of unl0kr without osk-sdl ever having been installed 
could either:

     a. Do nothing, leaving the package installed in a dormant state as 
it is now.

     b. Prompt loudly using debconf then automatically attempt to 
configure (this is somewhat recommended against in debconf's docs).

     c. Just automatically attempt to configure (negating the need for 2).


I'm somewhat reticent to do 3c as this will break installs that are 
non-standard (say someone's configured a TPM or yubikey unlock), but 
there is at least some desire for the package automatically configuring 
the system: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1028554


That leaves the matter of how do we trigger the switch? Currently the 
only packages installed indicating FDE on Mobian devices are unl0kr and 
osk-sdl. I can't think of a neat way to cause one to be removed and 
replaced with the other without triggering the install on non-FDE devices.