Escape % in $CMD variable

Fixes formatting on some rare cases when a percent ends up
in the $CMD variable, like these below:

- When assigning a variable, $CMD ends up with the second parameter;
in this case, $CMD will contain '+%s%N', messing with the syntax:

 $ a=`date +%s%N`

- A function (or command in general) that contains a percent symbol:

 $ to\%() { echo $(( $1 * 100 / $3))\% } # $CMD=to%()
 $ to% 2 of 10 # $CMD=to%