Fixes to prevent lockup of CHDK when video is recording and CHDK attempts to perform file IO.
http://chdk.setepontos.com/index.php?topic=9986.0
set CAM_IS_VID_REC_WORKS for a540, d10 and ixus140_elph130
Remove CAM_STARTUP_CRASH_FILE_OPEN_FIX (not needed after implementing file IO semaphore fixes).
ixus140_elph130 - set CAM_HAS_VIDEO_BUTTON
add get_video_recording script function, returns true if recording video (is_video_recording value). Unlike get_movie_status which returns platform dependent movie_status. Probably unreliable on some cameras: http://chdk.setepontos.com/index.php?topic=9986.msg113394#msg113394