Don't hardcode fps for fake screen

Currently, when main hardware screen is powered-off, X server initializes fake screen's timer with 1 second update interval.

Streaming software like Nomachine or Vnc, as well as desktop input automation suffers from it, since it will forever be stuck on 1 fps until the display is turned back on.

This commit adds command line option -fakescreenfps that allows the user to change the default fake screen timer.

Signed-off-by: Baranin Alexander ismailsiege@gmail.com

Addresses #948

Inspired by xorg/driver/xf86-video-intel#133 (comment 328077)