Add support for starting X11 mode fullscreen.

src/conio/ciolib.c

src/conio/ciolib.h

src/conio/win32gdi.c

src/conio/x_cio.c

src/conio/x_cio.h

src/conio/x_events.c

src/syncterm/bbslist.c

src/syncterm/syncterm.c