hooks: add a snapd-service-generator

[mvo5]

The snapd snap is special and it needs to run outside of the regular snap confinement. To archive this core18 will create the snapd service/socket via a generator at boot based on the content of the installed snapd snap.

[mvo5]

With this in place snap list, install etc will work in a core18 system.

[mvo5]

@zyga Sure, how would you like this to be documented.

[mvo5]

This will probably evolve into a regular unit iinstead of a generator. The generator might still be a good step on the path towards the real thing. What we discussed during the standup was that we want a "snapd-startup.service" that will:

• try to find the right snapd snap (looking at current, looking at what it tried to use last, falling back to the seed)
• mount this snapd snap (if not mounted already)
• run a helper inside the snapd snap that can generate the needed units for snapd to start
• try to start the generated snapd snap unit(s), if that fails cleanup, go back to the starting point

[zyga]

@mvo5 I think I understand this better after the call today. Let's work towards the goal and I can write documentation as we move along.

[mvo5]

We will probably use a different approach, I close this for now.