Merge lp://qastaging/~mterry/unity8/mir-startup into lp://qastaging/unity8

Simplify our startup handling of Mir environment variables and set the right socket path in snappy.

Simplify our startup handling of Mir environment variables and set the right socket path in snappy.

The old code set up a bunch of Mir variables in the pre-start section, guarded by [ -n "$MIR_SOCKET" ]. This was designed to tell if we were running under Mir or X (we used to support both). Since that time, lightdm stopped setting MIR_SOCKET to tell us we were in Mir and started setting MIR_SERVER_HOST_SOCKET instead.

So for a while now, that whole Mir section was not being run at all. This didn't matter so much because at some point, the default value for the Mir client socket became what we were setting in that section anyway.

(We also grew a code block in the post-start that duplicated some of the never-run block in the pre-start.)

So you could think that maybe we could just drop that block entirely. However, now with snappy, we want to put our socket in the hardcoded path /run/user/XXX/mir_socket, not respecting XDG_RUNTIME_DIR. This is because the 'mir' interface specifies that path as the socket to use. And XDG_RUNTIME_DIR is a namespaced directory, not the normal /run/user/XXX/ path.

So I've added that logic, and restored the code block that is responsible for passing MIR_SOCKET onto the rest of the session.

(Oh and I also dropped the MIR_CLIENT_INPUT_RATE bit that seemed unused now -- I couldn't find reference to that variable in the Mir source anymore.)