make stills without thumbnails and EXIF tags to minimize jpeg size

to explicitly disable thumbnail provide option -th none
to explicitly disable all EXIF tags provide option -x none

The above change reduces JPEG size: thumbnail is about 20-24KB, EXIF tags about 1KB.

Useful when performing feed on low quality:
raspistill -w 320 -h 180 -awb shade -x none -th none -q 10 -l /home/pi/live/last.jpg -o /home/pi/live/live%04d.jpg -t 300000 -tl 1000

brings file sizes down from 33KB to 9KB.