snap: add support user-sessions from snaps

[mvo5]

The ubuntu-personal team asked about support for user sessions. It seems we have most pieces in place. Here is a minimal branch, mostly to start a discussion (for a final branch we will need a bunch more tests).

The strawman in this branch is to use:

name: snap
apps:
 daemon1:
  command: foo --daemon
  daemon: simple
  user-service: true

alternatively we could use something like: daemon-scope: {user,system} or just scope: {user,system}. Or something entirely differently, I'm open for ideas :)

[zyga]

Looks sensible on a quick look.

[vosst]

@mvo5 Thanks for the PR. I tend to like scope: {system, user} better than a bool flag. Mainly because user-service: false does not necessarily imply system-service.

[morphis]

@mvo5 Is this supposed to work on classic as well or just for the purpose of an all-snap system? Wondering otherwise how this would fit into the user session managed by upstart on 16.04.

[vosst]

@morphis that's an interesting point. We would need a systemd instance managed by upstart, much like in the trusty scenario. I'll take the action to investigate into Trusty's systemd and check on its support for user sessions.

[chipaca]

a bare scope might make people coming from mobile think of something else entirely. Otherwise agreed it's better than the boolean.

[zyga]

I've added the blocked tag based on discussion above about 14.04 systemd running as a helper for user sessions.

[niemeyer]

This is a pretty trivial change and has been blocked for quite a while on unrelated details. I'm closing it for now, but please reopen when we actually intend to perform the change.