Currently, in order to use headless java, the headless flag must be
specified when launching the VM: java -Djava.awt.headless=true

Judging by traffic on the java forums, and from bugs we have received, many
people find this confusing. It has recently been suggested that we should
default to headless java on unix and linux systems when the DISPLAY
variable is null.

Note: we should not default to headless when the DISPLAY is invalid.

Also, this heuristic should not override user settings: if the DISPLAY is null,
but headless java has been disabled at launch (-Djava.awt.headless=false)
we should not choose headless.