cec: added '-s' or '--single-command' to cec-client. starting cec-client with this...

cec: added '-s' or '--single-command' to cec-client. starting cec-client with this parameter executes a single command in cec-client and does not power on devices on startup and power them off on exit. this way, you can use cec-client in a shell script. since there's some handshaking involved at startup, this is not very fast. to execute a command and only display the result and any error/warning, execute it with RESULT=`echo command | cec-client -s -d 3` (on linux/osx, win32 batch files are a bit different)