* Rename user_notification_add() to user_notification_add_fmt()
* Add new user_notification_add() helper function
* Use xvasprintf() to replace user_notification_add_va()
* Make better use of helper functions in config.c
Since we need to restore the status flags anyway, there's nothing to
gain from emitting the user notifications after dup:ing the pts file
descriptor.
On the other hand, emitting user notifications *before* dup:ing means
we can still print error messages for e.g. write(3) errors.
* 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.
