Enable it if supported by default, let the user override it with
the existing terminal_synchronized ui option.
This should finalize work discussed on #4317
Synchronized output does not work well with various terminals
(including the linux console). It should also be unnecessary when
not going through a slow link.
This will eventually be removed if it is not proven to be useful
to some users.
This should reduce flicker, by avoiding transient states where
info/menu windows are not displayed, and paves the way for proper
diffing of the screen.
