Add a new `don't read back' flag for variables, to prevent reading their value

Add a new `don't read back' flag for variables, to prevent reading their value
after modifying them. Give this flag to `display.focus', since screen
switching is asynchronous, and reading back will return the screen we are
switching from if the switch has not completed yet.

Also, disallow -= and += syntax for display.focus, as it doesn't make any sense.