disable GUI password prompt if running headless

[ixcat]

Note: may be X specific

dj.conn() defaults to using GUI password prompt even if matlab is started 'headless' with -nodesktop.

if there is a display available, the GUI prompt is still displayed, however, if a display is not available,
the GUI prompt will silently hang.

should probably default to CLI prompt either:

a) when started headless
b) if display is not available
c) both (if this can be determined on all platforms e.g. no / empty DISPLAY on x, etc.)

(depending on desired behavior for -nodesktop when display is available)

[ixcat]

^ discussion of 'gui available but running nodesktop, should popup or not' resolved - will assume nodesktop means nogui.