Foot is a Wayland client and cannot be run outside of a Wayland
session. As such, it makes more sense to use $WAYLAND_SESSION instead
of $XDG_SESSION_ID in the default socket path since this makes it
clearer which Wayland session we belong to.
Closes#55.
We now create a copy of the config for each client, and updates it
with the values passed from the client.
Since we're not actually cloning it (and e.g. strdup() all strings
etc) we can't call conf_destroy() to free it, but need to free just
the strings we've replaced.
