* Rename user_warning to user_notification
* Add warning and error types (in addition to the existing deprecated)
* Simplify logic when emitting a user notification after forking; we
don't need to copy the notification data since we're in a new
process and have total control over that memory.
This is used when spawning the slave, to set its current working
directory just before we exec() the client.
In a regular foot instance, we set the cwd from getcwd().
In a foot server instance, each connecting client sends its cwd to the
server, and we use that.
