XDG_RUNTIME_DIR, etc

[John Lenton]

we're currently not exposing a runtime dir to snaps, and that together
with our private random tmpdirs mean we don't have a good story for a
snap shipping a service that creates a socket and a binary that wants
to communicate with that service over that socket; people are using
the data dir for now, but it's not ideal.

I think we should:

* remove the random component from the tmpdir (this has security
implications around the predictable tmpdir creation, but I think it's
doable?)

* set up a private per-package system runtime dir mount, similarly to
what we do for /tmp (ie make it a sub of /run/

* set up a private per-package user runtime dir mount, under
/run/user/$UID/package.etc

does this all make sense, or am i overthinking it?